Phone numbers are basically super cookie identifiers unless you make a new phone number for each account and use different aliases & maybe addresses for them too. They all sell into centralized information systems and create profiles about you that are very detailed, which includes banking, income and credit info. So yes, the phone number is incredibly valuable, especially a place like amazon that shows different prices to different users and who's recommendation engine drives a lot of sales.

Phone 2 factor is pretty much the only kind of 2 factor most people will accept, and for most people the phone number probably has better security than most people's emails, because most people reuse passwords, while with phones you had to do a special non-password effort for them.