Why can't champs do their jobs effectively too often?

It seems that a lot of the time champs can't do the job that their meant for as effectively as they should. Tanks can't tank, burst mages can't burst, etc. Champs not meant to tank end up better tanks than those meant to. Champs not defined by burst can burst but those whose sole purpose to do it can't always achieve it, assassins have to be absolutely busted to actually assassinate someone, ADC's perennially have to use itemization not meant for them to deal meaningful damage and then end up busted.

I think that a lot of this has to do with base damages, base stats, and runes. For example, tanks often have some of the worst base health stats in the game. Why? They are tuned around the idea that they will buy lots of health and resistances. If they had high base stats then they would be impossible to balance. Then champs who aren't meant to tank end up with higher base defenses, buy lots of resistances and health then become an automatically more effective tank. Then they can rely on base damages to still do their original job.

Defense items in particular are too effective on characters not meant to use them. Tank items are built to be effective on everyone, which I believe is an inherent problem in defensive itemization in particular. Question: Why is it that there are very few champs who scale off of defensive stats? I know part of the answer, there will almost always be mixed damage in each game. Prime example, it feels not great to buy MR on Malphite when he scales off of armor.

There will always be a problem with tanks when they don't scale off of defensive items This is one that I have an idea of how to maybe fix. Make champion base stats have inherent scaling. Using Malphite as an example again, let his skills scaling continue to be off of armor and AP but make it so that his base armor and MR scale extra with each point of armor and MR he buys. He can retain his identity as an armor stacker who excels in teamfights and against high AD teams. This can be done to lesser or greater extents based on how much a champ is meant to retain a certain job. Those who are primarily meant to be tanks can have greater base stat scaling with defensive items and champs meant to live in the fray, bruisers, can have just a little. This way tanks can always be the most effective at tanking. A champ can still build tanky, it just won't allow for champs like ekko, fizz, etc. to be busted on builds not meant for them. Conversely champs who are meant to do damage can have base stats scale extra off of offensive items. This would mostly help burst mages.

Now I don't know if this could feasibly work and I do realize that it could possibly discourage off meta builds which is preferable, but its a possibility. Let me know what you guys think.