I absolutely agree with you but I think that's more of an ideal situation. There's realistically only a minute and a half to make these decisions though, often times much much less. If people are asking you to switch off of something because a second healer is needed and then you don't switch at all, it becomes a waste of time to argue the merits of a specific character, especially when Symmetra needs that extra time specifically to set turrets up and make use of maximum charges before time starts.

So when most people see Symmetra being picked and they voice their objections, saying "No, it's okay, I'm super good at Symmetra" misses the point. In this specific case it's boiled down to the idea of Symmetra being more of a defense hero than a support on most team comps, and when most teams are using 2supp/2dps/2tank, all that playing Symmetra really means is that the person stuck playing support doesn't really want to heal, they just want to play a variation on DPS that has shields and teleporters. It doesn't really help the team on an objective level with all things being as equal as possible (which it's designed to be in competitive), especially when at grandmaster there should definitely be at least two people on each team that can play at least one support. (I mean, hopefully, c'mon, play a healer, people.) Arguing about Symmetra doesn't help the situation in any way at this point, it just wastes time.

Obviously I don't know the whole story and I don't want to defend salty assholes, but it doesn't help that in the very limited time of vital team communication to just say "I can't be confident in using the other healers and it will work with Symmetra."