Most people have terrrrrrrrrrrible password practices. Absolutely abysmal. And they are keeping extremely sensitive data inside their email as well as using their email address as the key to accessing many other websites and pieces of sensitive data. Think about how screwed you'd be if you lost access to you email address. Now think of how many people DAILY probably loose such access. Phone numbers are an easy, mostly reliable way to identify someone and give them access back to their email address without an investigation and without human intervention.
Plus, let's just get reasonable for one minute here. Google already has your phone number. Ever given it to someone via gmail? Someone else ever given your phone number to one of their friends through gmail? They're just asking for permission to send you a text.