Is it possible to make a 3d sculpture in an hour?
Is it possible for you? We don't know. Different people have different abilities, and what's possible for one artist may be difficult or impossible for another. People also have different levels of experience, and the best 3d modeller in the world may not be able to match your 2d capabilities; perhaps you couldn't create a 3d model in one hour now, but you might (or might not) be able to learn to do so.
You also need to take in to account the desired quality and style of the end-product. There's a huge difference between a simple 3d model and a very complex and detailed one, and the same variation is also present in 2d art.
How long is a piece of string?
how much time does it take to make a 3d sculpture? How much more time to rig it?
It depends both on the specifics of the sculpture in question, and on the skills of the artist in question. Going back to your example time of one hour, a talented artist with good tools can create quite a complex and detailed model in that time -- but again, it very much depends on the specifics of the situation.
In some cases the code for loading and display of 2d art might be simpler. The hardware on the target platform (think mobile devices) might be better able to render a sufficient number of 2d assets at a desired frame-rate than 3d. Depending on the formats, you might be able to store your 2d assets in less space than 3d. Some teams have access to a skilled 2d artist, but not a skilled 3d artist.
I don't see the advantage to making 2D art for games.
Note that the depending on the situation the opposite of every one of these examples might also be true.
It's always a trade-off, and there is no single choice that is better in all situations -- you need to look at the specifics of your own situation and make a decision about what works for you.