I think the primary reason is because it's just cheese cheese cheese and a bit of more cheese in online play. People just want to "pwn nubs" rather than have a fun game so I'm not really that keen on 1v1 online.
Part of the problem for me is that many people playing competetively seem to disregard a
random opponent on the internet as a person, but instead treat others as if they were merely AI opponents with a slightly higher skill (or lower if the competetive player is winning). Some of these people often also have an inflated sense of their own skill level, possibly having constructed an elaborate tale about what they expect to accomplish (likely after viewing some videos of either a pro player or someone failing lots).
When they fail to win it's not their own fault, rather they got cheated by the system/their faceless opponent. Losing with grace is not part of their reaction.
If you then put those very same players up against someone they can put a face on, their reactions become much more dignified.