Why would max level be much higher, or higher at all? That would result in your Demigod having many more skill points available than in multiplayer, so SP builds would have no correlation at all to MP. More importantly you would be able to get skill combinations that you wouldn't be able to get in multiplayer, or you'd have to have entirely different skill trees.
It'd double (or more) the balancing work to get it done right and you end up with SP players wondering why MP is so gimped, etc.
I'm not seeing where the benefit would be, and you're not explaining why you think they'd do it that way. I can't think of any games where the fundamental gameplay changes that much between MP and SP. I'm not saying they don't exist, but they'd be the exception rather than the rule...