Tracking back a little bit on this one.
Why must magic items do more numbers? So they're attractive to players to take instead of just the playing the numbers game. I ran into this mostly in the 3e family of D&D, specifically PF1. It's hard to get PCs off the Big 6 (magic weapon, magic armor, cloak of resistance, etc) when they have choices of magic items. It's honestly hard to give up a +1 bonus to all of your saves and AC in return for sticking to walls and casting webs (cloak of arachnidia, for example). And that's because you know that saving throw/AC bonus is going to be consistently useful, unlike the items that do more cool things.
My response in 3e was to treat rings and cloaks, the items most crunched by the decision between cool effect and consistent bonus, as requiring at least a +1 deflection/resistance bonus like weapons and armor needed at least a +1 before loading on other enchantments. Give away BOTH a bonus AND a cool effect and players are a lot more willing to take the cool item.
The other solution is to simply remove all bonus items, and I'm not sure I'm keen on it. There are good reasons there should be items that give a numeric bonus to saving throws at the very least. Most PCs have 2 strong and 4 relatively weak saves. A numeric bonus on those weak saves goes a long way and becomes highly coveted.