Disconnect instead of resining, very annoying

why do people do this ?

Well, there might be some cases when people disconnect instead of resign:

case1: when the opponent is in a winning position and have many advantages and suddenly you found a forced checkmate combination or (his or her) queen is lost, the opponent will get annoyed and disconnect from the game, which is actually a bad thing 

case2: when your opponent network is bad, the game will be abandoned.

case3: when people start the game with zero interest and the game was not going in their strategy, people often left the game instead of resigning.

case4:By mistake closing the tab may lose the connection sometimes.

I hope I clarified your question to the best of my knowledge.

Thanks for your feedback, I feel I often experience case 1 however I take your point that there are other situations.

Why is it annoying?

It's their time. If they would be thinking in that time, it wouldn't be a problem, right?
Also, people have bad Internet connection.
I've played games where the opponent is desperately trying to fix their internet and continue the game, but chess.com automatically resigns their game, because "it's impolite to disconnect."

Because they're jerks.

Resin on the mouse makes it soften. Best to avoid.