The thing is, compromise doesn’t always start out as effort. Early on in a relationship (or maybe youth is a factor?) it can feel easy to work with differences. And then over the years as we grow and mature sometimes our wants, needs and habits evolve and set firmer. Our world view can change, our life goals can change. But you might still love the person you’re with so you continue to try and make it work. Sometimes that works out and the relationships stays strong, sometimes it doesn’t. I think if I was starting over again at my age I too would be a lot less flexible in what I wanted in a partner. I know myself better.
The other reason why people might stay in non-perfect relationships is that the way our society works, it’s centred around a couple being the primary relationship unit. So doing things like housing, child rearing, holiday companion, someone to help if you’re sick, someone to chat to if you had a bad day at work, etc, are all often done with a partner. Without a partner it can be hard work to build a network that can fill those roles, or you have to be happy to do it on your own. So yes there’s compromises, but if the partners value the companionship enough they may choose to tolerate the annoyances and personality differences.
I guess the smoking example is a good one. I really hate it, but maybe could live with it if other things ok. I couldn’t live with major ideological or political differences.