the combo would be bulky, require batteries and have a breakable bulb. that’s why. they’d wind up with so many customer complaints from normal use. if bulb dies or is on its way out, how’d you know if water was safe enough? diferent water has diff levels ofcstuf to kill, how do you know it’s been on long enough?
Bulb: LED.
Power: generator attached to pump handle.
Exposure time: simple statistics of microorganism survival time vs. UV exposure.
No different from problems solved by current filters & purifiers; just a matter of repackaging into a minimal unit.