There are a few reasons.
First, "a" soil analysis is often a number of analyses run separately with all of the results being reported on one form. The sample gets split into parts and on part will go over "there" for pH and salinity while another part will go over "there" for minor elements while another part needs to go over "there"....etc. And each analysis simply requires a minimum amount of material to get the job done.
Another reason is that the lab wants to be sure that the sample is at least of a certain size so that they can feel some degree of confidence that it's representative.
It's also just a fact of life that sometimes things go haywire during the testing and you have to run a procedure over again to confirm the results. If you only had "exactly" enough to start out with, re-runs are out of the quesion.
And finally, no, it's not likely going to be feasible for the lab to "re-assemble' your sample and return it to you.

2 cups, or 1 pint of soil is what labs need to do a good test of your soil. When you sample the area to be tested you take some soil from many areas and put that into a pail, bucket, container, and mix it all together, removing any stoners or large bits of organic matter. From that, very often, gallon of soil you send the lab one pint, or 2 cups, a representative sample.
When I was a Master Gardener and advised people how to take a soil sample we never suggested they save any of the soil in case a "retest" was needed, and they do not do that today.
Some of the tests are destructive to your soil so you will not get any back and would not want any back. As you correct the soil, adding organci matter, you will be putting more back then you took out.

we never suggested they save any of the soil in case a "retest" was needed, and they do not do that today.

It was never said that the client will be asked to hang on to extra soil but, trust me on this, the lab does if they have enough. That, as I explained in painstaking detail, is one of the reasons they ask for a certain amount.

Fascinating. Thank you both!

Interestingly, I did call the lab and they let me get away with 1 cup after all, but I can definitely see your point as to why they'd want extra around in case of any retest needs. Next time I'll be more generous, now that you've helped me better visualize a bit of the tedious background!

I test our soil in the spring and in the fall, around the blueberry shrubs. Since these are scattered about the yard, I typically do 12 tests per year. Each test requires one tablespoon of soil, and the procedure involves saturating the soil sample with de-ionized water. This is a pH test, I do not find it necessary to test for anything else. The samples go right back on the ground when I am done. Of course, they are basically mud, after I have added enough water to drain off 5 ml for testing.