Very simple. In a game that is based around loot and leveling up, it's silly to think you could keep the same gear you got at level 1, 10 or 20, all the way til level 100. That and if they didn't force you to get new gear all the time developers would actually have to put waaaaaaaaaaaaaaaaay more effort into their game to develop engaging, lasting content. Otherwise you would get bored much faster since you are keeping mostly the same weapons and armour all the time and have little need to hunt for new ones unless on the off hand chance they so happen to turn out to be better stat wise. But hey, this is real life, companies want to make money. So in order to keep you engaged and interested to keep playing in their world at a cheaper price to them they do it this way. Otherwise, quite honestly as I said, developers would need to create much more unique content to keep players interested in buying any future dlc or microtransactions. Which would also cost them much more to do. In fact Short answer, companies like money. That's why.