“If cholov yisrael spoils more quickly, it’s because the stores are negligent in how they keep it.”
AND because most cy companies are not using the more modern higher temperature pasturization. And some of us are glad of that.
The standard pasturization is fine for the purpose of killing dangerous bacteria that may be present. The higher temperature is only for shelf life.
Some cy companies are using the higher temps though, so seek them out if you prefer longer shelf life over less cooked milk.