Because at some point someone said "if we have multiple ways to get in contact with our customer, we'll be able to tell them about problems with their order faster." And then it became industry standard, and everyone upstream of the order also wants a phone number, and so if you don't put a mandatory phone number field in your form anymore, all the other ecommerce developers will laugh at you and call you mean names.

I don't know for other people but in my area the delivery person almost always calls me before dropping the package to check if I'm home and ready to receive the package.

My local Chinese place, which only allows pickup when ordering online and only takes cash, not only requires a phone number; it requires that the phone number be verified via a texted PIN.

For every transaction.