Why the fuck are we so invested in them?
Generally, politicians rarely have much impact on the way a country functions. They rarely interfere, and that's usually a good thing. Even presidents are normally purely ornamental. In my country, he makes about as much as an average engineer and is just a face. You need qualified politicians only when shit hits the fan, so they are competent to adjust things accordingly with policies or timely reactions to extraordinary things by moving resources between the public institutions and budgets. An exceptionally competent team of politicians who care about their legacy will also try to push actually useful changes that will help the country in the long run, as they know what they're doing. That might happen when you have a good economist making economic changes, good industry consultants specializing in policy, and good ministers who recognize their work and make it real.
Most of the time though, major politicians are good at staying in politics but aren't qualified to do the actual jobs, so the complaints are usually that they are getting paid for nothing, but at least they're not messing anything up.
When things are relatively stable, the problems begin when there's a wildcard politician who fucks things up just for the sake of it, usually through a mix of egocentric sociopathic behaviors and incompetence, when everything would be otherwise fine (or at least fine in the areas he interferes with).
Now it's rather obvious that America has entered a problematic stage. To the largest extent that's the result of regression in the levels of knowledge and fewer qualified brains coming in, to some extent that's the way the culture went, to some extent the poor overall health, wealth gaps, poor public wealth allocation, the demise of the business culture centered around meritocracy which made the country so successful economically etc.
It certainly isn't helping that so many other places in the world are doing things right - they are opening up and uniting, which creates a synergy effect, compounding their economic and quality of life gains. The EU became a larger economy than the US and the economies of the historically poorer regions there skyrocketed, with the most developed countries still growing faster than the US and enjoying a higher quality of life. At the current pace, China is bound to overtake the US economically in less than 7 years, reaping benefits of open trade, investment opportunities, and improving levels of education. In the meantime, the US is becoming more and more isolated with their problems.
But now to a lot of people, it appears that those problems now also come with a face, and that face is representing their country. Never have there been a president embodying as many negative stereotypes about Americans, who is at the same time also on the wrong side of so many problems over there. While even he will do some things right and have his supporters zeroed in on those things, for a lot of the more patriotic folks over there, it's probably hard to deal with the fact that their country is not only going through problems that are only getting compounded, but also becoming a public embarrassment, and there are people who support that, and that it will be difficult to undo. At least that's how I see it. When you see that your country is entering a "make it or break it" time of its history, and you believe that the person leading that change is dramatically fucking things up, you get understandably frustrated. I think it's different than regular dissatisfaction with politicians.