Short answer - because it's much harder sci-fi than normal Stellaris.
The long answer has numerous factors. Some technologies are more advanced than the game possesses (genetic engineering, aeroponics, red lasers, railguns, space habitation) because they are already ones that we are working with today and - like most sci-fi - Stellaris just hasn't kept up with cutting edge space science or the reasonably foreseeable extrapolations from that, especially in the context of another century or more of research and development and funding in the context of economic growth delivered from space development and the potential of weak-AI industrial integration.
As far as bombarding Earth goes, a projectile of sufficient mass to not vaporise from atmospheric friction and retain sufficient momentum to be a useful bombardment weapon wouldn't be able to be fired at the velocities these pellets are fired at with the heat dissipation technologies that cool the reactor and the craft, and therefore wouldn't be suitable as an anti-ship or anti-projectile weapon over the distances these weapons are being utilised. They would have to get orbital bombardment specialised ships to low Earth orbit. Likewise, Luna has supply issues based on the materials on the Moon that restrict the ability to fire other kinds of weaponry; low volatiles and minimal recoverable materials for nuclear fission or fusion weapons limits their locally mined and manufactured options.
As far as bombarding Luna goes, it would be much easier thanks to the negligible atmosphere, and presents a credible threat even with the more limited weapons available, and is why Lunese independence didn't take place until the War of Three Billion and the resulting loss of Earth's nations ability to dominate the colonies.
For Earth to regain that capability means being able to put very large warships into orbit, which presents a problem. If you look at the delta-v map in the Documentation post, you can see a Lunese vessel needs 1800-2000 m/s delta-v (allowing a little excess for non-perfect launches and a slightly higher orbit) to be orbiting and therefore able to fire. Earth however, needs 9500-10,000m/s delta-v to be in an equivalent position. I.e. an Earth launched ship needs five times as much velocity, which means a far higher fuel/ship mass ratio.
This means on a like-for-like comparison that a Lunese launched ship can devote much more mass to it's armour, it's electronic warfare capabilities, it's flares, it's ammunition and most importantly when dealing with railguns on these scales for this kind of mission profile, weapon mass.
Another Lunese advantage and disadvantage is that being stationary relative to the Earth's perspective opens opportunities for both sides, which allows Luna to do really powerful ground-based weapons that don't have to be limited by radiating the heat from power generation away.
And lastly, geostationary orbit is just under 36,000 km above Earth. Luna is more than ten times further out. Accurate bombardment from that kind of range is really rather hard; even with barrels at hundreds of metres long with ten metre diameters, feasible ranges in combat are pretty much exclusively in the sub-2000 km distances against other ships with the ability to dodge incoming fire. To use these weapons from these distances through the atmosphere would have a difficult time hitting the right city from geostationary orbit, and the right country from Luna.
Plus as you point out , because of these accuracy issues the residents of low Earth orbit, up to geostationary orbit and cis-Lunese space would all take a rather dim view of Earth and Luna firing artillery at each other; it would not be difficult to aim at an incoming ship and take out a habitat (as inherently only being capable of station-keeping, they can't get out of the way) instead with the crossfire.
As a result, warfare is kind of limited, leading to the characterisation as a second Cold War, and why the peace has lasted for so long; they want these weapons for the threat potential, and not so much for usage.