hella generalizing here but the vast majority of people who say this have never actually watched one
I agree from the standpoint of judging an anime solely by its genre, but mechs are just uninteresting to some people. Sure, an anime that includes or is centered around mechs can be good, but if you're not interested in mechs, it's not going to be the mechs that make it enjoyable. Not for me, anyway.
you really have to be in some depressed state of "i wish my life were something else" to really get into slice of life. that or you have the aptitude to appreciate character dynamics/dialogue over an exciting premise/setting.
i really feel like this a lot, sometimes i can barely watch an episode and feel like stuff when i'm only halfway through and sometimes i feel like i could just watch SoL forever. it just depends on what i feel like.
I feel like I'm in some semblance of that state all the time. Maybe that's what I'm missing.