I'll attempt to give you an honest answer.

Sombra is not a bad pick. Sombra is really good in a team where people are competent and coordinated, but she's also like Bastion and Widowmaker in that she's one of those characters that requires you to build a comp around them. Not everyone is comfortable with that because that means five other players will have to potentially play in a style they're not comfortable with or with characters they are not comfortable with.

Also,

my Sombra-only alt account

This is a huge reason. A lot of people, like you, make smurf/alternative accounts just to ONLY play Sombra. This leads to a lot of toxicity because there will always be situations where another character would be a better choice, but there are people like you who ONLY play one character and selfishly force people to work to make your pick viable.

Additionally, while you may consider yourself the exception, remember that there is a lot of bad Sombra players out there as well. I cannot remember the last time I ran into a game and felt Sombra was giving us a hard time. 99% of the time, I see a Sombra when someone is tilted and throwing the game.