Two reasons.

One, because this is the internet, not face-to-face interaction. You have time to communicate without stumbling on words or being incredibly upset. There’s no excuse for swearing.

Secondly, I run a family friendly page. I operate under the assumption that some of the ponies following me don’t know those words and have no reason to be exposed to them. Plus, it shows a certain level of maturity. And swearing isn’t the way mature adults communicate in a casual interaction.

I swear, but not online, and not in front of children. When Gilda tossed me like a rag doll into a wall, I swore quite a few times. Not directed at anypony, just as a reaction to having the wind knocked out of me.