[pixl97]

The most likely reason has been replied to multiple times, that the phone system doesn't pass it.

But another reason is this is a failsafe to ensure the right order number was entered, as with a phone system there are very few means of authentication before that point.

[NonNefarious]

Mmmm, not really. First of all, they system generally DOES check to see if the account number is valid. After that, all it has to do (and generally does) is ask for another piece of correlated information.

[Spooky23]

Also, some flows may use the user provided data, others do not.

People will do weird shit to get what the want, including using other peoples info at the IVR or asking for things that will get them to a human faster.