I think some people have an issue managing clones / phantasms or really understanding what they're about and how they're meant to be managed. They are a huge part of a Mesmer's damage and control - although I do find Mesmer takes a bit of extra awareness and positionin, more so than other ranged classes.
But I find that to be a general problem with people in this game and I'm not sure it's the idiotic default binds of the F1-4 keys scaring them off ever using them or some other factor, but a lot of players just ignoring their class abilities.
I've seen warriors never bother with adrenaline abilities, thieves who never steal, guardians who never use the AoE Aegis/heal (for themselves or if they see a champion about to squish an ally or something) and ofc in this instance mesmers who seem to treat their illusions as some kind of passive to be ignored... in any case they're all seriously gimping themselves beyond belief.