Because most agents will not accept such a listing contract.

That's one of the purposes of the MLS (Multiple Listing Service);

The listing agreement includes the listing agent's offer to pay commission to a buyer's agent.

I deal with property in another area where this does happen. You go to each agent giving them the listing, some ask for an exclusive, but I've never given it. Some will ask for a higher commission, if they cooperate with another agent, that will only happen with an out of area agent, not a local.
The important thing is to keep the same price for all, it is funny to the same property listed for different prices.
Just last week I discussed selling my property with several agents, first commission rates given ranged from 2.5 to 8%. The lowest has worked with me many times and knows my property, the highest requested an exclusive and does high-end marketing. When I did email the high-end agent saying I didn't want to list, he quickly cam back asking to negotiate.
I can't see that happening in my local area, just too many agents and they work on the MLS.

"Just trying to understand the reasoning behind why you can't have contracts with multiple agents to sell your home?"

There are 3 standard types of listing agreements, only under the Open Agreement can you use more than one agent.
(unless modified by an attorney and agreed to by multiple agents)

EXCLUSIVE RIGHT-TO-SELL LISTING CONTRACT
An exclusive right-to-sell listing is the most commonly utilized instrument. It gives the broker the exclusive right to earn a commission by representing the owner and bringing a buyer, either through another brokerage or directly. The owner pays both the listing and selling broker fees. The owner cannot sell the property herself without paying a commission, unless an exception is noted in the contract.

EXCLUSIVE BROKERAGE/AGENCY LISTING CONTRACT
An exclusive agency listing is similar to an open listing except the major difference is the broker will represent the owner. The owner still reserves the right to sell the property herself and not pay a commission. The broker is free to cooperate with another brokerage, meaning the second brokerage could bring an able buyer whose offer the owner accepts. Typically, the broker is paid a listing commission that is shared with the selling broker, so the owner pays both fees.

OPEN LISTING CONTRACT
An open listing lets an owner sell her home by herself. It is a non-exclusive agreement, meaning the owner may execute open listings with more than one real estate broker and pay only the broker who brings an able buyer whose offer the owner accepts.
The big difference is an owner will probably pay only a selling broker's commission, which is about one-half of typical fees. The reason is because the owner is unrepresented. Therefore, owners do not pay a broker to represent the owner, but do pay the broker to represent the buyer. However, if the owner finds the buyer herself, the owner will not owe anybody a commission. Open listings are not popular with many full service RE brokers.

berniek, I think the OP is just questioning the process, not asking what members of the NAR subscribe to, and what is customary.
In practical terms, I can't imagine, listing something in Los Angeles by going to every agent. My purpose for listing with an agent is to get exposure, if I just list with one agent my exposure is drastically reduced. I might as well put an ad in the newspaper.
My experience as mentioned in my previous post is not in California, in fact it isn't in the USofA. Things are certainly done differently in other places.

if I just list with one agent my exposure is drastically reduced. I might as well put an ad in the newspaper.

Actually, I think the opposite is correct. By listing with one agent, that agent will advertise and expose the listing including the MLS system which is open to ALL agents with an offer to co broke the listing. WHen you list with multiple agents in an open listing situation which is what the OP is referring to. One agent doesnt own the listing and therefore no one advertises or exposes the listing. The exposure is limited because only a handful of agents know about it as opposed to an exclusive right to sell listing which will allow the listing agent to get it out there to MLS and comitt to advertising it and exposing it.

if I just list with one agent my exposure is drastically reduced. I might as well put an ad in the newspaper.

Actually, I think the opposite is correct. By listing with one agent, that agent will advertise and expose the listing including the MLS system which is open to ALL agents with an offer to co broke the listing. WHen you list with multiple agents in an open listing situation which is what the OP is referring to. One agent doesnt own the listing and therefore no one advertises or exposes the listing. The exposure is limited because only a handful of agents know about it as opposed to an exclusive right to sell listing which will allow the listing agent to get it out there to MLS and comitt to advertising it and exposing it.

Confusion here, I was not referring to listing with one agent in the MLS. I was referring to listing with one agent not in the MLS. I do understand the difference. Note, I said I can't imagine listing in Los Angeles by going to every agent as I have done in other places.

Cmarlin, thanks for clarifying, What you're saying makes more sense now. I was also trying to explain to the OP the benefits of having one agent working for you who is part of the MLS.. (Not an exclusive listing)

"berniek, I think the OP is just questioning the process, not asking what members of the NAR subscribe to, and what is customary."

Like I said, the contracts offered are the contracts offered by the state regulating agencies. If you want something different, hire an attorney to write one for you.
NAR has nothing to do with it.

Like I said, the contracts offered are the contracts offered by the state regulating agencies. If you want something different, hire an attorney to write one for you.
NAR has nothing to do with it.
Okay... I give up.