system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Trek] Why don't we see any R-bombs, relativistic weapons?

Seems like a biggish ship loaded with rocks could warp near a target and be sent off at full impulse to slam into a planet and create a new asteroid belt.

No life support or anything, just shields, thrust, and mass.

You also wouldn't have to hit an inhabited planet. Blow up Mars and Venus, say, and Earth'd pretty much have to roll over.
gpt
I'd say its likely because M class planets are valuable.  Terraforming other planets is possible, but that takes time and effort, so capturing already inhabitable planets is far more desireable.  Otherwise, the the more aggressive 'Trek powers might be blowing up planets and stars left and right, since the tech is there for it (the Genesis torpedo, Soren's trilithium torpedo, Bashir-changeling's proto-matter bomb). 

With weapons tech like that available, MAD probably plays a role as well, since the level of AI available allows for automated second strike capability. So if you start blowing up someone's planets, they are likely able to return in kind.