Why can't I send photos with the short message function of vivoX20?

When sending short messages in the information software of vivo X20, if the short messages contain media files such as pictures and videos, they will be sent in the form of multimedia messages. In addition, when sending MMS, you need to use the data traffic of the mobile phone (the data traffic will be automatically turned on when sending MMS), and you can't use WiFi to send MMS.