Why do celebrities feel they have to buy a big house, when it's just a family of 2 or 4?

I wondered the same thing, but watched an interview with a professionaly basketball player once who made a lot of sense. He said that they get homes with basketball courts, movie theaters, bowling alleys, swimming pools, etc... to keep from having to go in public so often. Sometimes they want to spend quality time with their families doing things normal familes like to do, this way they aren't being harassed for autographs or followed around by papparazzi. As far as the bedrooms and stuff, I don't know why they need so many, but I can see why someone famous wouldn't want to be in public so much.

Some celebrities entertain a lot as part of what they do. If they head up charitable causes, sometimes they have fund-raisers at their homes. If they have another business-- say, a fashion design line of clothing or cosmetics or something-- they also might have to entertain as part of the promotion of their product line.

Some celebrities also espouse politcal causes. Barbra Streisand uses her home to hold fundraisers for candidates she endorses, as do other celebrities.

In my neck of the woods here in SoCal, when you read the real estate ads for the multi-million dollar homes that are up for sale, almost all of them trumpet how suitable the home is for entertainment. (FWIW, a lot of company CEO's and other business types whose businesses benefit from networking own large, ostentatious homes that they use for entertaining clients.

It's not just celebrities - I once saw an aerial view of a man involved in the medical industry (not a doctor) and his estate was the size of Michael Jordan's - and there's just this old man and his wife. It's all about status symbol - mine is bigger than your type of thing.

Just to be show offs!

They are Americans and can spend their money how they want.

because they want to show off their wealth

Because they can.