Because it is easier for people to paint you as a bad person than have their ideas challenged. We see it a lot today with words like nazi and racist that have lost so much meaning as they are hurled as argument enders a lot of the time. It appears that the old saying to "agree to disagree" is just no longer an option.

"I'm losing the discussion [Why am I seeing it as some sort of contest that I have to win? Not sure!] so I have to somehow hurt my 'opponent'. What should I do? Ah, name calling! I'll call them a premature ejaculator! That'll hurt them and I'll feel good when I see them hurt."

Immaturity. An absence of knowledge in anything other than their specialised subject. Being unable to even contemplate that they could be wrong. The inability to form their thoughts into a cohesive structure to counter an argument. Feeling inadequate, inarticulate and/or belittled. Fear of being ridiculed. A general lack of empathy.

When the subject is hot and we are emotionally involved things get rough, quick, dirty and easy.

Frustration from feeling that they’re losing the argument.

For fun

Lack of arguments

Because they are so weak.

Some people (like BEN) lack any sort of maturity and therefore can't accept the possibility of being wrong. It's a defense mechanism. I'd also wager that he's probably struggling with low self-esteem issues, so he relies on ad hominem attacks to preserve some semblance of dignity.

"You attack ME with facts, I attack YOU with insults!"

It's the same thing you see in relationships...The vast majority of people simply do not know how to argue productively, it's not something that's really taught anywhere unless you were some debate nerd, Poindexter 😛 anyway to them getting the other person to shut down is tantamount to 'winning' and some will do so by any means necessary

I dunno....coz ur a dumdum?

That's when I hit em with the "ok but you smell"

Frustration, bad life, bad environment, .

Because it's an easy device? Because they're frustrated that they're not winning the debate or argument because their ego is tied in with their belief that they're right (as it is in the vast majority of people), so they resort to demonizing the other party or trying to attack them instead of the argument (ad hominem) because it's all they can do..