Sensors that spit out text? Who in their right mind would want that?
SCADA grabs sensor readings from the underlying control system, most likely running on some PLCs, where you have to do calculations on the data in order to feed back control values to the process being controlled.
Now, a PLC is, admittedly, sort of like the general purpose CPU's dumb brother, and the instructions it accepts are rather limited. But, for a number of reasons, they're immensely suited for their task. The single most important one being the ability to safely and easily change a program that is in production. This feature is important because control systems often have to be tuned when they're commisioned, they don't just work out of the box. You have to fiddle with constants in order to get it working, perhaps even change the structure of the control algorithm.
Because we control engineers have to fiddle with the program while it is running, we really don't want to do string to int/float/whatever conversion and the reverse when working on the PLCs. That would just be yet another place where we could scew up horribly, causing a country wide electrical blackout in the proces. It's hard enough as it is, so keep it simple, stupid!