This is kind of an interesting thought for me, because it isn't immediately obvious why this is a problem.
First consider it from the point of view of the player, a player who uses bombardment to nuke worlds is at a serious disadvantage because recolonizing every world would be expensive and you lose the bonuses of capturing population. Moreso, if you bomb once, you gain what appears to be a permanent negative penalty from that race, which makes them harder to keep happy, which also means you require more garrisons which makes it harder to build up enough troops to avoid using bombs. In short, it makes sense from the POV of the player never to use bombing and just brute force capturing planets with sheer numbers of troops. It might be a little expensive to build up a mass of ground units, but once you do, you usually don't lose many capturing new worlds, so there's only a big initial investment. Using bombing not only hurts your future productivity, but it also makes holding on to captured worlds expensive and might require recolonization if you decide to go for scorched earth rather than ¨soften em up then invade¨ bombing tactics.
For the AI it's kinda a different story. They usually have a LOT of colony ships, are often fighting a losing war, and it benefits them to scorch your worlds rather than try to conquer because you might come back and reconquer them. The AI does sometimes use troops, but only on lightly populated worlds. On populated worlds, it will nuke everything to the ground rather than try to invade, which can be pretty crippling to you as the player. It will also send small raiding fleets to hit vulnerable planets, which is a tactic I can't use as I can't commit a troop force to a throwaway attack, but rather can only concentrate them in one or two big fleets.
But, here's the thing, The AI doesn't ONLY fight with you. It also fights other AI. When that happens, the AI's tactics often result in both sides losing massive amounts of population and worlds. If both sides are scorching each other, by the time they are done and agree to end the war, they've both taken massive damage. If the AI were more hesitant to bomb worlds, they'd do less damage to each other. This means that, while there's some benefit to the AI to use scorched earth bombing tactics on you the player because it hurts you more than pure conquest... it definitely hurts the AI in any war with another AI because both of them are scorching each other.
It's almost like a prisoners dilemma, as a player, I don't bomb, so I risk taking more damage if I let the AI near my worlds, while in the short term, the need to use troops makes my victory against the AI harder, even if, in the long term, I benefit more. The AI's short sighted use of bombs is really bad when two AI go to war though, as it basically ensures that the AI never benefits much from fighting another AI in the long term, since both sides will likely lose a lot of population, which in the long term greatly weakens the AI's late-game power overall.