Messages using email instead of phone number
A single contact entry in your iPhone may have one or more email address, phone number or both listed.

People using iPhone are logged in with their Apple ID which requires an email address to create. The phone number of the iPhone gets linked to the Apple ID. However, Apple ID can be used to login into Mac or an iPad as well. It's also possible that a person is using a Mac or iPad and not having an iPhone, in which case the phone number doesn't get linked with Apple ID.

Messages app on iPhone gives priority to iMessage over SMS (if the recipient's email/phone number is registered with Messages). The Messages app displays the email address of the recipient if it's an Apple ID. The message can be sent as an iMessage.

In your case, its possible that the contact has only email address available and it's an Apple ID registered with Messages. If the contact also has a phone number saved, it will be listed and SMS can be sent to it.