Right. A single starship can devastate a planet's surface, but it's hard to do that when you're also fending off an attack. Also, I would submit that the Federation (& to an extent, the Klingons, Romulans, & Cardassians) have rules of war to kill people in a nice way that doesn't harm their little ethics. This is why the Klingons went apeshit over Genesis; why the idea of the biogenic weapons on Celtris III tightened Starfleet's sphincter; why the use of said biogenics by the Maquis (& later Sisko) was such a shock; why the Son'a using subspace weapons freaked out...Geordi, was it? eventhough they never signed the treaty; why the Jem'Hadar blowing the shit out of lifeboats, adding anticoagulants to their weapons, & basically being bioengineered drug-addict cannon fodder was so demoralizing.