Brecht wrote the initial Cycles implementation during the time he was away from the Blender Foundation working for some other (who's name I forget) renderer company -- so, basically, for free.

Back when I used to hack on the low-hanging Blender fruit my impression of the core devs was they did it for the love of doing it, any one of them could've gotten a job paying a lot more but they always stuck around because they liked it. And that was when they could only afford a couple fulltime devs who I imagine weren't making a whole lot.

Also, they couldn't pay anyone enough to mess with the spaghetti code of the old builtin renderer...