I suspect the reason was the same reason he didn’t want to generally take board seats (vs being a direct advisor).
You don’t get to play your own game entirely anymore.
Being on a board of directors has a host of legal and interpersonal and business obligations that are fairly far removed from being a mentor to a CEO.
Being a GP in a venture fund, maybe, 10% of the job is being a mentor to CEOs. A lot of the rest is running an investment firm. Sourcing investments, dealing with fundraising and LPs, dealing with investment decisions re: other people’s investments, dealing with running a partnership, etc.
If you can make enough money doing what you want, probably best not to do a job where only a minority of it aligns with your interests.