Is it? Can you elaborate? I think if you have a SaaS with happy 100 customers paying 10$ each per month it is easier to scale up to 10k customers than if you only have 2 customers to get to 200.

Software is the worst. After 15 years of open source development, I have basically earned nothing

Well you have this at roughly $2000 total: https://play.google.com/store/apps/details?id=net.sarasarasa...

And this at almost a million dollars: https://play.google.com/store/apps/details?id=com.modelmaker...

I'm not sure if these are outliers, and my "random" sampling is just picking out highly visible apps. But I've made good money off an app built in a month, and I've made relatively little from a book written in 3 months.

Software has a lot more variety. You can make say, a communist themed productivity app vs a forest themed one and these will appeal to different people.

No. Software follow the same pattern.