All of this is so that they can mine phone numbers for older accounts. Now they require a phone number by default.
It makes sense to add a way to recover access to your account if you fear that you may forget your password. It makes no sense to give google your phone number just in case they may out of nowhere decide that you look suspicious (for using firefox? moving to another country? something else?) and reject authenticating you despite being able to enter your password correctly.
"you should not have used their service for your whole life's activities anyway"
You should not do that in general as Google is extremely unreliable. I had given them my phone number for my account but this did not help. I posted about it at https://news.ycombinator.com/item?id=30544030 a few months ago.