Maybe I’m wrong but I think the idea of “friend zone” implies romantic rejection, whereas being actual friends is just a platonic relationship with no romantic aspects. Friend zone is like emotional attachment that is getting in the way of a genuine friendship because of that disparity. It sounds immature to say it if you think of it that way. It’s reminiscent of the incels on 4chan for a lot of people too, so that doesn’t help. Maybe that will clarify why some people think it’s cringey to say ‘friend zoned’?