Same reason that you can't cancel a gym membership by walking up to a person and saying "cancel my membership".
Predation, pure and simple.
Google doesn't care, no one is holding them responsible for predatory behavior. It's profitable to steal from your customers and there's no downside to doing so.
Where do you live that consumer protection is this bad? Of course you can cancel any gym membership at their physical location, as long as you can also sign up at these physical locations.
What idiot lawmaker would allow a system where you can sign up without a computer, phone or fax, but then need these media to cancel what you just signed up for?
Many memberships (like phone and internet service contracts) aren't rolling contracts, they're annual or biennial to lock you in. Of course you can cancel at the term of the contract, but many start strong, let it peter off, and by they time they realise they're paying for a service they're not using, they're well out of a cooling-off period.
It may have changed but there are (or used to be) gym memberships that would only let you cancel if you could prove that you moved and your new primary residence was more than one hour from the nearest branch. (I'm pretty sure there was a law passed to end this practice, but it was widespread)
In the United States this is not true. Bally's gym memberships are examples of this. I had one once and there were only three ways to cancel it. 1. You die, 2. You become disabled in a permanent way that would prevent you from ever using it, or 3. You move far enough away that there is no physical location within 100 miles of you AND they won't be building one within the next year.
This has changed since I had mine as I had the membership around 30 years ago, but they still in general do not allow cancellation at the physical location. You must write to them.
Edit: I just read that they all transitioned to LA Fitness now, and I'm not sure what their policies are but Bally's, while unethical and infamous, were not illegal as far as I know.
Predation, pure and simple.
Google doesn't care, no one is holding them responsible for predatory behavior. It's profitable to steal from your customers and there's no downside to doing so.