It's always the bigger studios who utterly mess up in making an actually playable game, which indicates that the problem is not something inherent but a simple product of laziness and greed. (Latest example: see Creative Assembly's meltdown)
Best case study is Mojang, the company has over 800 employees but it is literally outperformed in game design and update quality/quantity by ten people at Re-Logic. (which includes managers and legal as well!)
Are you serious? Teams have increased massively. Super Mario Kart for example had less than 20 people working on it. That's not even the size of the audio department for many modern AAA games
I enjoy both games, especially Terraria, which I have played since its original release. But let's not lie to ourselves that the volume of content updates for Terraria is anywhere close to the updates that Minecraft has received. Adding content for a 2D game is a lot easier than adding content for a 3D game, even if you're using voxels.
You don't see the thousands of indie games that were never released because the creators screwed it up, or they ran out of money.
Or the indie studios who never made a second game because they didn't recoup their costs.
Indies that fail just fold and you never hear about them. AAA that fail release their games.
Things like this happen because the people giving you money have a hard deadline and you ship what you have.
Or you decide to use a game engine that was more difficult to use than expected.
It's insulting to say "well, just make the game better first, duh". I promise you, they know.
But they have to balance a lot of things you don't see.
If you ever find yourself saying "why don't they do [obvious thing]", stop and assume you don't have all the facts.