Google & Facebook capturing Mobile numbers

In the recent months I have seen stepping up of efforts by Google & Facebook to capture mobile numbers of their users. The message is quite simple: in case your account gets hacked or you forget the password you can regain control because we have your mobile phone details. Sounds very logical but I have two very simple questions:

If the account gets hacked how secure will be the mobile no?
Why can’t Google/Facebook ask for alternative email id instead of mobile no?

While Google/Facebook are capturing mobile numbers no one is thinking about potential spamming and unwanted calls that you will receive if the data privacy is compromised.