Sometimes it’s really a compatibility issue that is the root cause. A “couple” can start out as FWB, but they actually have nothing much in common except physical attraction. Obviously, that’s fine for FWB. But it can become problematic if it evolves into an actual LTR.
So the chick ends up seeking what she is “lacking” (from her relationship) from the friendzone guy friend. He doesn’t have the looks, but he might offer her things like attention, deeper conversation, emotional support, and interest in certain activities that her bf doesn’t have, etc. In short, he offers the “beta” side of the relationship.*
Not saying it’s right or proper, but it does happen. Sadly and oddly, it’s the second guy that is keeping their relationship together. Kick him to the curb, and the “relationship” will fall apart. (The chick is technically playing both guys or “balancing on a two legged stool.”)
*I’m talking about a more extreme case where she seems TOO close to a guy friend. I have nothing against opposite sex friends per se.
TLDR: If she’s so happy in her relationship, why does she need lots of time/attention from the guy friend anyway?