Rpg with flexible holy trinity [closed]

Most RPGs work out like this:

You are a mage, but the moment you pick up a shield and a mace you become an handicapped mage that can’t cast spells, so you are forced to play out as a glass cannon or a supporter.

But why?
What stops an intelligent sentient creature from learning more than on thing?

In real life people can be intelligent, strong, fast and a good artist because usually people have a long life span, long enough to give them the chance to learn any skill they wish.

Therefore it makes no sense that a mage can’t be a tank, what stops the mage from learning defensive abilities and taunts? Or what stops a mage from changing career during their life time? Clearly it is hard to find a person in real life that has done one and only one job during their entire existence.

Or what if one wants to be a “pyromancer” or “pyronom” and instead of throwing fire balls from a distance they grab their enemies face and ignite their palm to turn their face into charcoal?

Are there any games with a completely free class system?