Same reasons as World War Z.
That shit rots right off, and most people died in bed (naked or in underwear). When people do have clothes on, it’s often minimal.
I assume that, when the infection hit, it was nighttime in the area the game takes place. Not only that, but the people in the evac shelter weren’t there for, what, a day- weeks at least, and water, wind, dirt, etc. is going to take off those clothes if they’re not already gnawed off in the first place.