I am pretty sure that none of Bill Gates, Larry Ellison and Elon Musk could get a 900
850 is the max for the scale people generally refer to when talking about credit scores. Googling around, some banks seem to use internally a different score scale, but let's set that aside for a sec.
People can, and in fact do get perfect score. If you understand exactly how it works, its' not that difficult. It has very little to do with how much money you make, and is a pretty artificial metric.
When you get a report and it says things like "too many accounts with balances open", it doesn't mean "you have too many accounts with balances open". It just means you don't have -precisely- the amount of accounts the algorithm uses for a perfect score, so you lost a non-zero amount of points for it, and since you said you have a mortgage, it's probably what it's referring to.
To get a perfect score, you need a bunch of accounts open, that were opened several years ago (none recently), that are used but have 0 balance at the moment they were audited. Your available credit across those account has to be very high, and you need multiple accounts from different credit providers. There are a few other factors, but if you do it just right its pretty simple, given enough time, to manipulate your credit to get a perfect score.
In fact, some people make a game out of it. The only gotcha is you have to use those accounts sometimes but they have to be at 0 or nearly 0 the moment they're reported, and you never know when that will be (since it can change). So often you'll hover between 845 and 850 (or whatever other scale you're looking at, though those may have slightly different criterias)