Honestly it's because Joff isn't talented at drawing and he isn't talented at writing either. If it was just one of the two he could hire someone to do the other half, but that's impossible.
Geoph has exactly 1 actual talent and that's making music. Unfortunately though, with everything he creates the level of popularity will always be inversely correlated to the quality of said creation. He's terrible at QC, yet it's by far his most popular work. Alice Grove was just meh, but he didn't get many people to care about it at all. His music is actually genuinely nice, but nobody gives a shit about it. I doubt that even half of his patreon superfans ever heard it at all.
That has to fuck with his head. Imagine all the low effort garbage you put online gains a huge following but every time you out effort into something you're actually proud of it gets completely ignored. I'd start doubting my own judgment enough to put me off experimenting with new shit.
Besides, many things can be said about Hef but there's no denying he works hard. Especially compared to other webcomic artists, he has always frequently and consistently uploaded. He did a different comic and his music stuff all the while never skipping a beat on the daily updates for the original comic. It's not a good comic, and I don't think much effort goes into it, but the dedication at least is admirable. So in the end, I can't blame him for not doing even more projects on the side. He draws a crappy comic every single day and that's something I could probably never do.