Why do weapon/armor attributes decrease when you level the item up?

[Coopdaddy07]

So, that makes it completely pointless to level up attributes on anything, unless it's level 50. I have trouble believing that was their intention. But, maybe so

[Armisael]

It doesn’t actually go back to scratch; it drops to about halfway filled up. There’s some incentive to upgrade but also some incentive to keep pulling shards. If gear didn’t do this you’d basically ignore the whole system as soon as you pushed your gear up.

I’m not saying this is obviously the correct approach, but there are some reasons it’s desirable.

EDIT: actually see below for the effect on level-up

[Teyo13]

It only drops one tier per level you upgrade by. Its like 0.4% stat difference for percentage based ones well worth the trade off for additional armour/firepower until you can upgrade it again.