I have a serious question.

Why are there so many stories in which the CMC have a catapult?

Many stories.

Either as a sub plot, or as just a mentioning.

Though, it is concentrated in more older fics, some more recent fics also have the same recurring theme.

I would make a list, but it is very late, and I'm lazy.

Comments

[Zasyri]

Other than it being an easily accepted device for them to try and use, in Vol.10 of the official MLF:FiM comic, the CMC brutalize Ponyville with fireworks during Summer Wrap-Up using a catapult to rain hellfire down upon a hardware store.

[mintgreenconspiracy]

I think Jan used one in one of his early Ask Cruaders as well (and if it's good enough for him... )