Why won’t they?

In my experience, it has been smooth in the past.

It’s more likely that ad networks are showing you ads based on the other person being in vicinity and having things in common. I don’t think voice snooping is the main cause here

If the advertisers start paying less, then Google will just increase the number of ads shown to the user. And playing content on more tabs won’t even matter to them.

The issue is that they are a monopoly so they control and they can do whatever they want. The alternative is federated video services but those aren’t matured yet.