I don't know why businesses don't just do this. At least, those who want to deal with customer calls in good faith.

In Canada there's a healthcare support line where you can call and talk to a nurse, like if you have something going on where you're not sure if you should go to the ER or just wait it out. They get pretty busy, so when the wait is going to be a while, they'll have you enter a callback number and then just give you a call when it's your turn.

Then they don't need to have phone line capacity for all active agents plus everyone in queue (which can be much higher than the number of agents), they just need capacity for agents plus everyone going through the process to get added to the list plus everyone who decides to just wait instead.

And I don't need to sit around giving my attention to my phone any time their hold music gets to a lull or they play a patronizing recording with PR speak about important my call is to them.

Not technology but economics I suspect. No one wanted to pay for it. Which would make sense if Google is trying this since they have money to throw at things.