The main reason is that, if you already have a build and a set of armour that you like, you won't ever want to chase something else. New items and mods might make you want to change your build, but most sensible people store armour that could be useful in the future, so you'd just be pulling out stuff you already have. The same is true for weapons; why would someone want to search for a new loadout when they could just use Mountaintop and Recluse for all eternity? You may not like that reason, but you're going to like it even less if armour sunsetting is undone and you start to think "there's no incentive for me to get any of this", especially with Transmog confirmed for the near future. Realistically, you only need 3 pieces of good armour. Class items don't have stats and you're likely using an Exotic armour piece, the stats of which don't matter a huge amount. That gives you a year to find 3 armour pieces with good stats in the slots you need. It's not as bad as some people make it out to be.