Dave said during the run up to Horizons that they could've released planetary landings sooner but they did it when they felt confident that they could implement it well.
But... (in my opinion) they didn't implement it well.
They implemented it, technically.
You could land on planets, and drive around but.... it was a height map- with frankly, very little on it. (there's a quote from Braben saying this is exactly what he wanted to avoid!)
They added some things later (thargoid bases and gaurdian stuff)
but... the bases you could assault were always very poorly designed gameplay wise - even though it LOOKED like they had ideas for them, that just seemingly never got fully implemented.
Those "skimmer control" data links? be nice if that swapped the skimmers over to your side, or allowed you to "neural link" (original term for using the turret on the SRV) into them and control individual ones - activating others things, shooting at other skimmers.
There's a load of other things like that, that seem to either do nothing, or do something so unintuitive or poorly telegraphed, no one really cares.
It was also designed without really thinking about the fact people had ships... It was like the developers assumed people would just use the SRV because... why wouldn't you? the SRV is a cool new thing!!! (not realising players are fully aware their ship has massive missiles and stuff and can just nuke everything from orbit - so they are gonna do that)
(Also Remember the blue circle scanner thing for ships which dissapears the closer you get to the ground, so that you HAVE to get out and use the SRV's wave scanner? - the wave scanner is cool! but... why do your ships sensors get less accurate the closer you get to something... - it's purely to force use of the SRV, not a clever design in which SRVs are useful because of what they are...)
The turrets "auto repair" after a while (with poor telegraphing of it happening leading to unintuitive situations) - skimmers were changed to instantly drop ship shields and knock out thrusters if they were rammed as a quick fix to patch over a poor design idea, some turrets were made exceedingly powerful against ship shields to dissuade players from using ships to assault bases (ignoring the fact this would have been immense fun in multiplayer to have air support - and could have been great if multicrew hadn't been botched regarding SRV use) in order to force them into the SRV so they could get the "intended experience"
It's almost as though the design team get given a single specific thing to design, and aren't asked to take the rest of the game into account when they make stuff. I'm sure it was a great design if it was purely an SRV game - but it aint.
So... I mean, I guess what I'm saying is.
Ok...