It's a success for Figma's founders/investors. It's a failure for everybody else.
Maybe the lure of several billion is too much to resist.
Well yes seems obvious, unless they already had that kind of money. It's easy to say that from the sidelines, since I assume you haven't turned away several billion of dollars
Short of a slightly bigger house, I already have difficulty imagining what I’d use more money for (aside from start another similar company, which seems kind of pointless).
Another good reason to exit is also just to rest, take some time and enjoy the world a bit after being deep in founder mode for so long.