I don't think it's ALL about the money, though I can't help thinking that plays a big part - perhaps mostly for those studios who think a good / proven director will help them milk the most out of the franchise and put money, pressure and/or promises to bear to get such directors on board.
I think to the directors the challenge has to be a big part of it. At least I hope that's why Ang Lee took on the Hulk (a film that I didn't appreciate much when I finally saw it.) The challenge of operating within strict frames of plot and well-known OTT characters and action scene requirements and all that, satisfying all those conventions and limits, being subjected to the scrutiny of fans who in many cases already live and breathe this world and the characters too - and yet manage to come up with something original, expressive, interesting and meaningful. And fun.
It's like the old Norse poets who had to write within so strict rules for syntax, syllables, rhythm and rhymes, required imagery, required symbolism etc. that it's a marvel they didn't all write the same poem. Yet some of the best are true and unique masterpieces.
