People are asshats. That’s why

It’s all about cost reduction and fraud prevention. Those cheap stickers are simply tamper-proof, getting easily destroyed upon removal. Prevents being resticked onto other items. A simple example would be thiefes who remove discount labels or price tags from cheap items and put those onto expensive items, ready to argue that the item must’ve been mislabled by the store and then insist on the low price. Doesn’t work with a tampered sticker.

Also, there actually are easy-to-remove labels being used - just not for pricing out stuff.