I think there are a few problems with seamless space-to-planet transitions for KH.
- KH worlds have a super abstract, simplified design when seen from space
- sometimes KH worlds will break reality with how they start, placing Sora and pals in a predicament (falling into the rabbit hole, suddenly becoming sea creatures, cold opening on a crappy raft in the middle of the ocean) where seeing them land with the ship would only limit these possibilities
- Some KH worlds are super linear, some are not, but in both cases you risk spoiling the coolest vistas and reveals by giving an immediate bird's eye view of the playable zone
I do think a lot of these could be alleviated with a more esoteric 'approaching the world' sequence, but I fear without a hard cut, it's tough to integrate without sacrificing some of KH's most advantageous 'cheats'. It's a super cool effect in games about more traditional Sci fi space travel tho