They're not operating at cottage industry scale, they're talking hundreds of acres I believe. Those portable mills are more of a cottage/craft industry (I had one come and mill some of my trees a few years ago)

I suspect owning a mill exposes you to additional risks as well. I know here in Canada a bunch of the softwood mills in B.C. closed a decade ago, I suspect it's big time boom and bust and now you've got expensive equipment you're in debt for...