Why is the mobile phone not the default SMS?

Because his mobile phone number is not enabled with imessage or his mobile phone number is not the receiving object of imessage, you can't send his mobile phone number imessage but his ID imessage.

Apple's priority is imessage. When the sender has the imessage function, it will send imessage first. After closing imessage, your friend will send you a text message.

Of course, there is no facetime icon next to his number on your mobile phone, otherwise you can't stop using your mobile phone number as the receiver of facetime.

To sum it up. Because he doesn't regard his mobile phone number as the recipient of imessage and facetime, you can't send him imessage or facetime wants to send a message to your friend, that is, after sending him imessage, hold down the message. At this time, there will be an option to send it as a message, so you can send him a message.