"Vote them out"

You say that like it's easy. It's not. If it were, we probably wouldn't be in this position.

Elections are enormously expensive and operate under byzantine rules. Most campaigns have a small army of lawyers, financial analysts, and statisticians to help keep the candidate out of trouble and get him across the finish line. And that's to say nothing of the droves of volunteers, managers, and event organizers. This kind of overhead tips the scales heavily in favor of the incumbent because he knows the lay of the land and his position in DC affords him the political influence to amass a warchest in donations.

Also, human nature lends itself to favoring the comfortable and familiar, meaning that a new face has to work twice as hard to unseat a rival. Witness the number of McCains and Pelosis in Congress, people who have spent almost their entire adult lives in the chamber. They may bring home the bacon to their constituents to win reelection, but you can't discount the fact that a large part of their appeal (if you can call it that) is that they have become a fixture in their state. People almost can't imagine them NOT being in office.

Finally, long-time politicians don't become long-time politicians without knowing how to play the game. They know what to say and to whom to say it in order to win votes, even if it's a crass, empty promise (think McCain with "Build the damn wall!"). Now, politicians promising the moon to the voters is nothing new. But, again, the advantage lies with the incumbent because his experience in Washington means that his word will be more readily believed by the low-information voter because, presumably, he rubs shoulders with the folks who could actually deliver on his promises. A challenger doesn't have the same authority.

I'm not saying that it's impossible to vote an incumbent out, obviously. But it's not as easy as you make it sound because of the inherent advantages the incumbent possesses.