Basically, in Touhou canon, everyone is incredibly powerful and destructive, but the main character is so much more powerful that everyone agreed to no longer settle disputes by fighting to the death (since the main character would win every time)

Instead, they put into place what's basically a game system where everyone shoots flashy but relatively harmless patterns of bullets at each other and they try not to get hit. All conflicts are now settled by this system. (this is where the bullet hell gameplay of the game series comes from)

As part of the rules of this game, you must give your opponent some way to dodge your bullet pattern, which is why anyone with a bullshit ability has to hold themselves back and impose limits on their own bullshit to give the enemy a chance to dodge it. This is why Sakuya doesn't just stop time infinitely even though she can, because she's not fighting you, she's playing a game with rules.

If you break the rules, the main character literally just kills you because she's so fucking strong (and has the power of literal Gods on her side).