Q - Can we build ground-based anti-space weapons on the planet?
A - Here are reasons why you can't do that in PROB.
1. For realistism, you will have to build ship-mounted weapons on the ground or even larger in size, in order to reach and damage spaceships. However, it takes your tiles which could have potential resource output, and may assign collateral damage to the colony when they fire if your colony is not well-shielded. You will also have to build multiple cannons in order to cover your fire in all directions, and only some of them can fire in a round. Therefore, this structure is less valuable than a static orbital defense station or a mobile spaceship.
2. They can be unfortunately destroyed, because in orbital bombardment event, which building to destroy is entirely random. Lacking of ability to protect themselves impacts their viability and made the game relying on your luck, messing up both side's game experience. On the other hand, if we make them indestructable, or at least last to be destroyed, the damage must be ignorable small, or this is sure to mess up the attacker's game experience, and if we do, the ground-based defensive weapons are entirely useless.
3. Though scriptable, event-based damages assigned to ships are fixed, they can't be evaded, shielded, or reduced with armor. If we admit that ground forces have arc emitters, so why does the fleet use the poor guns at the first place?

Q - If we can do everything with fleet, why do we need armies?
A - Yes, why? Stellaris designer has decided to make orbital bombardments soft and weak in order to make armies useful, but if we decide to fix the orbital bombardment, the offensive armies will of course become useless, and the defensive armies can do no more than slightly slowing down your step. I suggest you to forget about the armies.