CDRMITTENSThe space armament treaty says: no nuclear, biological or laser weapons in space. but kinetics...
AMillionNamesScience fiction always challenges my suspension of disbelief is when people land on planets to skirmish with conventional weapons instead of, say, throw a big heavy aerodynamic solid rock from space.
You can make good reasons or at least plausible ones. 40k has planetary shields. There could be a civil war so neither side might want to destroy the surfaces of each planet and civilians. Battletech has spaceship tech being rare and irreplacable, and there are treaties limiting orbital and nuclear bombardment.
Sometimes the most realistic route just isnt fun though so i can usually look past the contrivances.