Notch wanted to "create a Valve," not work for actual Valve
Years ago when Minecraft was still in beta -- only on computers back then, but still very popular -- Markus 'Notch' Persson was invited to visit Valve in Washington, and while there he received a job offer from the juggernaut publisher. He turned it down, and explained why in new book Minecraft: The Unlikely Tale of Markus 'Notch' Persson and the Game that Changed Everything.
"Somehow, I felt that Minecraft was maybe my chance to create a Valve, rather than work at Valve," Notch said about that encounter (via an excerpt on All Things D), which he didn't realize was a job interview until he was in the middle of it.
Mojang isn't a Valve yet, certainly, but Valve itself wasn't built in a day. One thing we probably do know, though, is that Notch sure doesn't need that Valve money these day.