See eg gmail: you can't set up 2fa without supplying a cell (you will be allowed to remove it later, but how many know to do this?) Your phone number is trivially stealable -- see eg youtube video of people just stealing phone numbers with a crying baby and a sob story. https://youtu.be/F78UdORll-Q?t=133
Also, lots and lots of places have trivial routes around 2fa because people losing their password and/or 2fa is an order of magnitude more common than theft.