Insecurities and delusional thinking, that gets in the way of rational thoughts.
I mean I'm not a doctor, these are just my observations.

To get reactions.

Sounds pretty common here

What a timely question. I was just explaining to some insecure crybabies elsewhere why some people do this.

For me, it's to elicit a reaction. I do it for 2 reasons.

1. Retaliation. Typically the poster will insult me either directly or indirectly. I retaliate and escalate.

2. Boundary testing. To push the person and see if the have the stamina to put up with me and keep up with me. If I insult you and your immediate reaction is to whine, shame and complain, you are a worthless opponent and have lost my respect.

I just like to piss people off.

I didn't receive any love during my childhood so bring sarcastic is my only way of emotional expression

Yes and an inferiority complex

most members here do that

Their underpants are too tight. Makes them cranky.

I don’t know fatty why do they?

Jk

Trolling perhaps?

Yea and meanness

Massive insecurity and bone-deep dissatisfaction with their own existence.

You're mom is insecure....Oooooooooooh!

They have nothing else to work with: intelligence, humility, character etc.

Trump has made being an obnoxious-dimwitted-bully kewl. Happy days!