Oh boy. Let's preface this wh
ooooole thing right off the bat by saying that comparing a full game two and a half years after release to a game still in the first phase of beta is simply asinine.
Terraria's underground caverns were better designed (from a platforming perspective), easier to locate and move between, had a large variety of monsters that made sense and were custom crafted to fight well in those locations, ores were balanced to promote underground exploring without making it frustrating, and constructs ranging from tiny underground huts to major dungeons made it fun.
Terraria's underground caverns weren't 'designed', the procedural generation process was just set up to have more empty caverns than Starbound. People on Reddit (who, of course, don't cite their sources, but if I had to guess it would be IRC) have said that this is still being tweaked and is not final.
The monsters themselves were also just inserted procedurally, and there wasn't 'large variety' of then. Slimes at the surface, different slimes, worms and skeletons in the underground, imps and bone dragons in hell. It took them a year before things like bats started to appear.
Ores weren't 'balanced', they were randomly generated in and, as I recall from early on, frustratingly scarce (especially on Large worlds).
Finally, those huts (actually called Underground Cabins) were added in 1.1, almost half a year after Terraria's launch. The dungeon has been there since beta, but it's also the only dominate feature like that in the game. Starbound already has stuff going on underground and lots of 'mini-dungeons' that just happen to be at the surface.
The main problems are as follows:
If I wanted to be a smart ass, my response would look like this;
1. Beta.
2. Beta.
3. Beta.
4. Beta.
While I am a smart ass, I'll actually address each point.
1. The goal of Starbound isn't to just dig down into one world, but to explore many worlds. Yes, you can skim the surface of a planet and get ore, but the large deposits are deeper underground. The dungeons also don't have ore, they often have pixels, tech and weaponry, which in the scheme of things are much more important than ore.
2. The randomly generated monsters, as you say, are a WIP, but they also have the potential to surprise more often than Terraria's "Oh look, a different colored slime" approach. Each new planet has new enemies that have new attacks. If you ignore the whole monster taming aspect that especially highlights why that's a good thing, there's also the fact that it will theoretically keep exploration fresh; you will never land on a planet, see a monster, and know exactly what it will do. In a game that's all about exploration, that's essential.
3. I've found all sorts of weird shit underground, from little sewer areas to shrines to these weird frog merchants. I've seen weird little brick areas with tiny houses in them, large underground mines full of tar, mini-biomes (slime, blood, bone). It's there, it's just not always apparent. I do agree that they need more stuff underground, but... well... beta.
4. See first paragraph of this post; tweaking the amount of empty caverns underground is in the works, and Terraria itself wasn't 'designed' to support platforming. For every example of decent platforming I can recall in Terraria , I can remember just as many times digging straight down and not finding anything. It's the nature of the procedurally-generated beast.