Those don't look like stains. They look like bleach spots, and there is no way to get rid of them.

Is it possible they were washed with something that had bleach on it? I'm thinking about a cleaning cloth or towel that had bleach on it. Maybe even something leftover from a prior load.

Some liquid detergents have bleach in the formula. I remember having this problem in the past when the liquid detergent dripped from the dispenser onto the dry clothes.  Once the detergent is mixed with water, you wouldn't see any marks but whatever was sitting on the top of the load got damaged

I've had that happen when I've washed a load directly after a bleached load (if you ever use bleach). I think the machines sometimes have bleach left in the dispenser or something, but Lyndee Lee's explanation seem logical, too. So, now I wash the bleach loads last and hopefully, it's a couple of days before I do another load - so any leftover bleach evaporates. If I must wash something right after the bleached load, I am sure it's nothing that bleach would affect (definitely no dark colors).
If those were my pants, since they are unwearable now, I would put them in a load with some bleach and see if you can even out the color. They will be whiter instead of beige, but should be wearable. I swear every piece of clothing (and even the last TOWELS) I've purchased recently, say to wash in cold water and don't bleach. I pretty much ignore that, especially for cottons.

Some questions come to mind...

1. How many times have the pants been previously washed? Or is this the first time washed?

2. Powder or liquid detergent used in this recent wash?

3. Water temperature in this recent wash?

4. Was detergent poured onto the laundry or put into a dispenser?

* * * I once ruined expensive fabric yardage by pouring liquid Tide on top of the fabric before the machine filled. The optical brighteners caused permanent streaks and the yardage was a complete loss. I never knew what happened until years later when I found this forum.