Great thread now what is not much mentioned so far but for me the most important factor is scale of Imperial Guard operations.
WW2 was the largest military operation in human history at its peak with a bit about over 10 million troops (both sides together and including noncombat troops) involved at one moment and along 3000km (1900 miles) front. Now from all I read the important conflicts in 40k are even larger.
If you have 2 Operation Barbarossa, 3 D-Days, 5 Vietnams and 12 Stalingrads all going on at the same time all over a giant planet where do you really shoot at with your Navy Ships?
It will probably be houndreds of thousands of square kilometers you have to shell, none of the enemy is concentrated but everything is spread out.
And your intelligence, surveillance plus precision are so bad you never know where exactly which troops are and what you hit.
Better use your navy ships to protect the troop transports getting more reinforcements. And for the enemy side the same better use ships to threaten troop transport ships the guardsmen are much more vulnerable inside ships than on the surface of a planet. If you have smaller invasions that aren't on a scale as described above the troops can even spread out better and you probably also have less naval assets.
Then you can't shoot forever. When you fire for days and weeks, ballistic macrobatteries need ammo, energy macrobatteries can overload and have issues needing replacement parts and such. And even plasma engines need fuel which is normally not an issue but what about if you 24/7 Sunsear a planet for extended periods of time? As said the naval assets might be better used in other ways especially getting more troops onto the planet.