I love building tank with full mr against a team of all ap

This sounds like something that needs more context.

This sounds like a healthy thing considering AP is countered both by HP and MR.

Riot needs to removed there % damage period. That is the problem.

I think we need a lot of context here. In general, all AP teams are incredibly weak. Most tanky champions can survive most magic damage from building 1 big MR item plus the HP from their other items. If a team is AP heavy and you build 3 big MR items, you generally become almost immune to damage... There are of course a few champions that are really strong (e.g. Azir) because they're sustained damage and not burst (and Void staff exists), but nonetheless you'll still be doing a lot better than if you were against that champion and needed to build armor too...

If anything this problem exists against all AD teams. I hate how the enemy team can be all AD, I can be even/ahead and be building only armor items, and still get blown up fairly reliably because of the sheer amount of armor shred and penetration in kits and items.