Probably because making sure that clients trust the right set of non-public CAs is currently too much of a pain in the ass. Possibly an underrated investment in the security of the internet would be inventing better solutions to make this process easier, the way Certbot made certificate renewal easier (though it'd be a harder problem as the environment is more heterogeneous). This might reduce the extent of conservative stakeholders crankily demanding that the public CA infrastructure accommodate their non-public-facing embedded systems that can't keep up with the constantly evolving security requirements that are part and parcel of existing on the public internet.
I don't see a reason why that should be a problem to solve for public CAs and rest of the internet? Complaining about multi-perspective validation or lifetime is silly if the hindrance is someone's own business needs and requirements.
The second side is that if it's so tedious to approve and install, use solutions that require neither. Surely you don't need to have some artisanal certificate installation process that involves a human if you already admit that stricter issuance reduces no risk of yours. Thus, simplify your processes.
There are automated solutions to pretty much all platforms both free and paid. Nginx has it, I just checked and Apache has a module for this as well. Could the author write a blog post about what's stopping them from adopting these solutions?
In the end I can think of *extremely* few and niche cases where any changes to a computer system are actually (human) time-consuming due to regulatory reasons that at the same time require public trust.