Well not because its not competitively viable. It’s just not meta right now, and it probably won’t be for a while because there’s a gentleman’s agreement in pro play not to use it, similar to the autosniper, although the autosniper is also partially because it can be a really bad time to drop it into your opponents hands.
In regular matchmaking it’s not too uncommon to see.
Of course, the meta is aks, m4s and deagles, but really, most weapons are compeititively viable, depending on the situation. The mp5 is criminally underrated. I wrote a reddit post during the Astralis era about how good dualies were on pistol rounds, and they’ve only just gone meta despite not having had any change to price or functionality. Same with the scoped rifles.
There’s one weapon in CS that’s not viable, and it’s the m249. All the other ones can be used competitively.