I don't see anyone else tweeting the amount of money they've spent on a hobby, a sport, a car or anything else at all.
Tweeting/posting your account balance with RSI is an activity and phenomena unique to Star Citizen whales.
If it isn't anyone else's business what you spend your money on, then I'd suggest not tweeting it in the first place.
But! The fact is this - your account with RSI shows your status in the game world hierarchy, that's why these guys tweet it. CIG have produced nothing that whales can use to publicise their status within the game, so they have to post and tweet the dollar value instead.
They should be tweeting and posting screenshots of their fully manned Idrises in action, but they can't because they don't exist, so they post the dollar amounts they've spent on them instead.
That's the short and simple forum post version, but I could probably write a thesis on it. It's very straightforward and highly documented stuff. It's worthwhile and satisfying to watch the Star Citizen crowdfunding phenomenon from this perspective. Pride of ownership gives way to pride of money spent.