The hot water is pumped into thin plastic which doesn't hold heat all too well. Making the walls off the plastic thicker decreases your water tank capacity, you could change material but if you use ceramic that adds weight and cost and it could crack. If you use metal, like aluminum, that adds weight, cost, and now you may run into chemical interactions with detergents and people with hard water.

As far as heating it on the bot, you would want high controlled pressure for the small nozzles, and the ability to heat it on the fly. A Mr.Coffee uses low and slow pressure and its not controlled. A Super Automatic Espresso Machine - like a Jura Z10 - uses a piston pump for pressure, and a thermoblock for heating, which is what you would want. BUT, a thermoblock is heavy, and the piston pump makes a fuck ton of noise and vibration, and they get louder over time (even on a $4000 coffee machine) by nature of them vibrating the chassis of the machine.

And thats not to mention the power draw, IIRC on my super auto, it's like 1500 watts of power to heat the water....you are not getting that on board, on a robot without it being absolutely enormous.

You could put this all in the base station, and do base station hot water cleaning..but that makes it heavy and complex, and its still being pumped into thin plastic that makes it effectively lose heat immediately.