You grow accustomed to whatever level you get and then you want more, it is human nature, call greed.
That would have been my first thought too, however my boss is a multi-millionaire, has been since he was in his 40s, is now in his mid 70s and has never been one for a lavish lifestyle. He is in the same house he moved into over 30 years ago, drives a 20 yr old Audi A8. But he is always into new business opportunities, and trying to drive forward his existing businesses.
So I think it is a few different things.
For sure some that come into money "easy", their lifestlye just goes totally crazy, if they have a house, they now need 3 houses in 3 different countries, and now only wear clothes the once. So now they need staff in all those houses, and multiple cars at each place, and likely a chauffeur And, Nectar island looks good lets do a month there, and a Yacht would be nice...but hold on that one there looks nicer....etc. And then think of where some footballers came from, a lot of them (not all of course), if they didn't have that natural ability, they would have been doing really low paid jobs or into criminality, so some came from bugger all in a financial sense, and it is really like winning the power ball lottery in the US. And if you think footballers are prime for this you are right, but then look at the literally £100s of Millions being thrown at a single golfer to go into the Saudi league. The nature of golf is that it's mostly people who came from good earning households (it costs much more money to get into golf than say be discovered as a footballer, basketball etc). And so the obvious "in yer face" display of wealth isn't there.
For others it is a sense of greed and it's all about the bigger and bigger bank balance, whether they spend it or not.
And some (like my boss), it's their thing, being successful is the driver, and successful is taking an idea, jumping over all the hurdles and setbacks to make it a profitable business, and then the next deal etc.It's what gets them up in the morning.