For sports shirts the advertising is literally part of the shirt design, if you remove all adverts with a razorblade now you'd wasted a lot of money on a shirt with huge holes in it.

A friend of mine used to do this with products that self-advertise when he was younger (I notice his Tilley hat is unmolested, presumably he had now stopped), but it doesn't really make sense on say a reproduction football kit.

Hah! Someone else who does that!

(Or at least I did, for a couple decades...)