Because it could get you banned.
I don’t watch streams so I’m not sure what they use, but there used to be a third party tool that just allows you to trick the client into “receiving” model changes from the server. I’m not sure if this worked by calling in-process functions or if it was a sort of man in the middle that actually injected network packets but it certainly wasn’t model editing the client files. Both of these methods would be easily detectable if they chose to do that.
Warden used to verify the integrity of at least some data/model files. This happened in reaction to hacks that altered those files. One example is the language file, you used to be able to replace it so that you could understand the other factions chat. I would suggest not editing any of the data files for that reason, even if it’s for ‘innocent’ changes.