Because you can send a message to a /person/ (or multiple people) instead of a /phone number/. Depending on said service, said person can interact with the message independent of device or phone number. Useful for people who interact with people who change phone numbers/devices often and/or travel between countries. Just send the message, and you don't have to think.
But people will forever stick to their stock messaging application as much as possible, sticking to slower, more restrictive technologies. MMS is a dinosaur.
And SMS is a dinosaur too (but it's still around and kicking) and it's still the best way to get mobile app installs.
Sorry, but the need to have the same app installed to send and receive an image significantly restricts the audience of a given product (and you're locked into someone else's app).
Current: Step 1 >> Get 100M users to install an app. Step 2 >> Get them to take pictures of stuff. Step 3 >> Get permissions all the people who took picture to upload the pictures or just replace their normal picture-taking behavior with your app. No biggie.
With universal MMS (via Twilio): Step 1 >> Get them to take pictures of stuff
To me, having to think about what service person 1 uses compared to person 2, etc... that's way harder than using phone numbers.
And besides, with those services, I am sending messages to usernames, not to people... except for Facebook, in which case I am sending to one of many thousands of John Does who are on Facebook.
But people will forever stick to their stock messaging application as much as possible, sticking to slower, more restrictive technologies. MMS is a dinosaur.