You haven't looked at too many keyboards, have you? Yes, you can bend notes on a keyboard. They have a thing called a pitch wheel.
MIDI is Musical Instrument Digitial Interface. A MIDI guitar (or a guitar run through a MIDI converter) just sends out a string of commands like "turn this note on", "turn this note off", "do this programmed command", etc. Any MIDI device: a guitar, a flute, a keyboard can drive the same MIDI device (a synthesizer, a sequencer, etc). That's kind of the whole point.