If you want a real answer, a big part of it has to do with consular non-reviewability. Basically, there is far less ability for an applicant to make legal challenges to visa decisions made by a US visa officer outside the US.

Another reason is, what happens if you apply for a renewal or different visa type while you're in the US, and your visa is denied? Now we're relying on you to leave the country, whereas if you already had to leave the country to apply and you're denied, you're not still in the US.

There actually was a pilot program for domestic revalidation of H-1Bs. Applicants liked it (no need for a trip outside the US), and those of us working in India liked it (less workload for us). However I doubt this administration will support expansion.

In my experience most renewing H-1Bs planned their visa interviews (or often "dropbox" cases where they didn't even need to come in person) to coincide during a few weeks trip home. They were not generally coming to India, then applying and waiting several months.

The cases that take months are those with some problem - missing some documentation, evidence of petitioner fraud, national security concerns with the applicant, etc. And yes, in those cases people (and sometimes their families) end up getting "stuck" outside the US, kids miss the start of the school year, people can't get back to their apartments and houses and pets. It sucks, but we have vetting for good reasons.