Why don't supports abilities scale off stats rather than just AP?

I'll use sona for an example

Her Q should still scale only off AP Her W should heal should scale off her HP while her shield scales off armor/mr Her E should scale off her bonus MS Her ult should still scale only off AP

Why doesn't it work this way instead of giving them all AP ratios? I feel it'll keep supports from being broken in other roles and strong in the support role. Normally i'm for breaking the meta but when I see a lulu top dominating it gets annoying or if I see a taric or braum 1v1ing someone.

Another example I could use is Janna. Her Q should still scale only off AP Her W should scale off her bonus MS(would synergize amazingly with her passive) Her E should scale off her armor/mr Her Ult's heal should also scale off her HP

This gives supports a lot more options as far as builds and build paths go. It also gives them a lot more utility based on how they build. At the moment for Sona or lulu or morgana or annie it's just build sighstone then go straight AP. If it's Leona/Braum/Alistar it's build sighstone then build the normal tank stuff that top laners build

With a change like this a support will have to question. What boots should I buy? Should I get more HP for heals or more MS to make people faster/slower? Maybe I will get more durability so my shields will be amazing.

As it stands it's ill get sighstone and build as much damages as possible. That nullifies the entire point of a support. Which could be one of the reasons mages are always going bot. Brand/Annie/Morgana/Vel'koz etc.