I think the reason we are seeing two candidates who are unpopular for reasons of mental fitness has to do with several forces at play, and it comes down to how power is gained and distributed in our system.

First, I am defining power as the ability to make people do things they would not do on their own volition.

In any system, the longer you stay there, the more opportunities there are to amass power. This power comes from connections you make. In my teenage years, I didn’t have much power for many reasons, but one was that I didn’t have powerful connections. All my friends were working retail or fast food or other summer jobs. With age, my connections are more powerful. My best friend works at Oracle and another at Microsoft. As a musician, I’ve been around the block a bit and now know folks who are producers and have some notariety. This is a big reason we are seeing more and more elderly folks in powerful positions. Not only do they have power, they have connections.

The classic cases of corruption we see from the outside, are people returning favors when viewed from the inside. To them it’s common decency. Imagine you were passionate about food and feeding people, and you started a restaurant or maybe a non-profit to feed the homeless. If someone you knew saw your work and believed in what you do, and they could give you a hefty low rate loan or been a donation, the decent thing to do would be to thank them, and if they asked for a favor down the line, you would feel terrible to not return the support they gave you when you were small and starting out.

Throw that in with a first past the post voting system that allows power to consolidate around two parties, and you’ll see the natural trend is for those with the most experience and connections to find their way to the top while closing more and more opportunities for younger folks to make their way in. The connections of the youth simply can’t compete with the connections of the elderly.

One part of your post that struck me was the “why do we allow”. Personally my take is that we shouldn’t make strict requirements of who someone is in order to run for office. There are other ways we can vet those unfit to serve. My concern with something like an iQ test or upper age limit is that we will be disenfranchising marginalized groups from taking power. I don’t mean to go full slippery slope, but I do think this is a dangerous precedent to set. Should an elderly person be barred from running for president? Should someone who has a mental illness like depression, bipolar disorder, ADHD, or schizophrenia be barred from holding office? If project 2025 takes place we may ask the question should trans or homosexual people be allowed to hold office? For the last one, the argument from conservatives would be that they are too “morally deficient” to hold office. Personally I find that repulsive.

By limiting demographics of who can and cannot run for office, we would also allow the possiblity of a rogue president to disqualify opponents if they can manipulate a test required to be eligible to run for president.

I think these are reasonable considerations when trying to answer your question. I hope this was helpful.

If you’d like to learn more on power structures, CGP grey made a great video that is a great tl;Dr to my first year of my polisci undergrad.