The cool part is, they don't need to. The founders are in control, not the VCs or other investors, so the company isn't obliged to put generating money over all other concerns. And being billionaires, the founders are probably doing fine. They'd apparently prefer to keep the good thing they've got going than to ruin it by selling it off.
There's more in life than getting even more money.
Long term that should net them a lot more money. Having and keeping a true mission will allow them to leave all competitors behind. Because even if competitors can seem strong from time to time (VS Code and Atom recently) they will end up dying because of de-prioritization and leadership without vision (Atom).
I think they aren't technically billionaires. Rather, JetBrains is valued at over a billion dollars. So if they sold their entire stake, then they'd be billionaires.
I'm sure they pay themselves well but I don't think they will be sitting on a billion dollars in either cash or shares.
There's more in life than getting even more money.