Planets kind of have the same problem as Japanese occupied islands had in WW2. When you can no longer command the seas or space, your enemy can concentrate far more firepower than you can. The Japanese, late in the war, saw at island after island that their island based aircraft were swatted from the skies by US carrier aircraft flying in considerably larger numbers. Although it won't necessarily be cheap, CIWS should still need maintenance and/or GSP for munitions, and they can't catch every missile. You can swamp them as a result, and all you need is enough of them to ruin the industrial base of the planet.
After that, the planet is effectively neutralized as a threat, as there's no way it can produce new stuff. If you plug up the jump points all you've got is an annoyance that can't do much. And if you've got a bunch of large, heavily shielded and armoured flying bricks with a couple of decent beam weapons you can plink away at the planet at a cost of MSP and fuel while the defender has to choose to risk their STO guns or to let them keep shooting.
There's simply an opportunity cost that renders ships a superior investment when it comes to defenses, so planetary defenses are unlikely to be very heavy compared to a peer's navy. The fortifications aren't there to render a planet unassailable, that's a pipe dream, but to render them too much of a resource sink to bother while the navy's still a factor in the system's defense.