The fundamental goal of voting isn't to pick the best candidate, but to pick a consensus candidate in such a way that everyone feels their voice was or at least could have been heard. The distinction gets obscured because a lot of people describe their decision about who to vote for in terms of candidate quality, but if you scratch the surface, you'll find that most have cultural and ideological expectations in mind, and because those expectations are often incompatible we have to find a way to balance them that everyone will agree is fair.

> I’m not allowed to vote medicine FDA approvals because I’m not a doctor.

Seriously? That's probably because the FDA does not have the power to declare war, annex territories, or sign treaties on your behalf.

> Why are some topics “restricted” to the experts?

You're still allowed to go across borders and get medication that the FDA has not approved for your own personal use. This "restriction" isn't nearly as complete as you pretend it is.

It actually only binds what professionals can do not what citizens can do.

> But voting for president is not?

It's actually /any/ representative. Does that make it clearer for you?