I noticed that CPMA players use Keel's model with the light green skin almost as the default for both the player and bot models, and this is what most of the YT videos covering CPMA matches have too. Is there any particular reason for that?

Keel's the biggest to represent the bounding box, doesn't have much animated motion and makes distinctive sounds. Ultimately, it's an audiovisual advantage.

IMHO i don't like that bad habit.
Edited 2.96 hours after the original posting.

It makes sense. Truth be told, I'm not a big fan of replacing everything with one just for the sake of gameplay. While it is a significant advantage for many talented cybersport quakers and speedrunners, it feels underwhelming to me when this trick is applied to regular matches that are recorded on video and published afterwards.

In my opinion, everyone playing with the same model and skin is like everyone playing with a highly increased picmip index — is only useful for specific relevant events, maybe testing.