Yes, there is lots of land available. The problem (I think) the OP was referring to is that most areas have building codes that regulate what kind of structure you can build.

Go far enough outside the cities and there are still many places in the US that don't have building codes, though. So you can go and build your very own neon green & pink hobbit hole with a tree for a doorway and no one can say anything :-)

There are huge areas away from the main coastal population centers, this is a really big continent. But you might have to live without easy connection to the grid, drill your own well, handle you own electricity generation, deal with some long distance wifi connectivity. But there are folks with their nearest neighbor several km away.

Where I grew up I would say building your own home is almost as popular as buying one. But I wouldn't say the land is "nice." You can even do this in places close to Silicon Valley, such as the Santa Cruz Mountains. Although, there can be a lot of restrictions on the land use due to environmental and other zoning regulations.

Yes, you can find inexpensive land in the Santa Cruz mountains, but you have to be a little careful. One common reason why the land is cheap is that there is no pre-existing well or other easy access to water. If you have to drill a well, it can cost many thousands of dollars, and there is no guarantee of success.