Why does US Customs & Immigration ask the purpose of my trip when I'm returning home?

You may not be refused entry (my understanding is that in the US the 5th amendment allows you not to say anything, but I'm not sure how far that goes - ie are you even 'in' the country at immigration?). However, the point of immigration is to ascertain whether there's anything suspicious about your entry. You may be 'just returning home', but are you bringing in drugs? Were you taking something over to the other country? Were you engaging in terrorist meetings? All the stuff that nobody is going to answer 'yes' to, but these are stuff they're trying to find out.

If you've been visiting a country, like, say, Cuba or North Korea, they're going to be intrigued - it's rare for a US citizen to visit them as a tourist, and they'll want to know your reasons, for national security / homeland security.

If you're inconsistent with your responses, or they don't like the responses (or you don't answer) they then are trained to flag you based on some criteria as potentially suspicious.

This would likely then lead to additional one-on-one questioning in a small room elsewhere, where they may require additional proof about what you've been doing and so on. Are you really just returning home? Where do you live? etc.

As for having a legal right to enter the country - that's a good question, but a better question to be asked separately, I think, as it's not directly related to the questioning about the purpose of your trip.

Since 1823, freedom of movement has been recognized as a constitutional right, so if you are a citizen of the United States and have a valid passport, you have the right to enter the US, full stop. Now obviously the droids at the border may not like it if you're not cooperative, but at the end of the day, yes, you can refuse to answer and they'll still have to let you in.

That said, there are a couple of random edge cases discussed at length here, eg. if you're a dual citizen who is held to have forfeited their US citizenship (used to happen reasonably often, but is virtually impossible to do by accident these days), are obviously drunk while attempting to cross (Customs will usually do you a favor and deny entry, since the alternative is to allow you in and immediately arrest you), etc.

While US citizens may not have to answer questions at the border your computer, cell phone and any other property does not have the same right to silence. They can be searched without a warrant or reasonable suspicion. They can make a copy of all your computer files, photos, text messages etc. for later review.

If you have data you don't want searched or copied the Electronic Freedom Foundation recommends not to bring it across the border but send it encrypted via the cloud or FedEx on USB drive instead. That may not stop the US government reading it in the long run but will avoid issues at the border.

Who might legitimately not want their data searched? Doctors with confidential patient medical records/photos, businessmen with confidential trade secrets, attorneys with confidential client notes are all good examples.

Another example is photographs of you and your under-18 girlfriend/boyfriend on your cell phone that could be construed as illegal.

This link claims that you have the right to remain silent, though this would probably cause a huge delay at border control. Sometimes its easier to follow through with reasonable immigration control demands, rather than spending extra time arguing. Which option to choose is up to you.