Because texts are NOT data. Texts are basically incoming phone calls.
Data connections are initiated always by the phone, share air time slots with other phones, and bypass most of the phone system, going straight to the internet. Because they are shared connections, data is always billed by data amount, not by connection time.
Incoming texts must use the telephone control system find your phone wherever it is in the world, make arrangements for billing and connections, then allocate a dedicated air slot on a tower and send you the page and get acknowledgement. So basically, they're billed like a phone call, which is usually minimally charged by a minute. How much do you pay per minute?
In addition, texts require an entire set of infrastructure for store and forward in case you're not online at the time.
So no, data and text are TOTALLY different things.
An advantage of texts is that, because they are just another call, the phone doesn't have to spend extra battery keeping a connection going to be watched. Texts are as true a push mechanism as you can get on a phone, and without battery penalty.