Apple is a retail business and they can deliver tech support at scale by doubling their retail locations into support centers.

It's unfair to compare a retail company's availability to a SaaS provider such as a Google. It would be wise for Google to start opening retail locations if it intends to get into the retail game for this very reason.

This isn't an unfair comparison at all. From the consumer's perspective both provide many similar services. And products, for that matter.

And (serious question) what's stopping Google from providing support, when other SaaS providers can (and do, profitably) and other ad/media companies can and do?

Specifically, paid support would be nice.

According to Steve Jobs' biography, this was a deliberate decision on their part, to better control the user experience.

They didn't need to become a retail company, but they understood that the customer would be better served that way.