You answered your own question. Not every device used in a home (remote control, light switch, thermostat) needs to be replaced with a full tablet computer, even if it could be.
Who would want this Amazon product for every light switch, remote control, and thermostat? No, most buyers will probably just buy one for a household. In that case, I agree that a regular tablet would suffice.
My point was that you have a remote control already; you don't use a tablet instead of that, just for use as a remote control. In a similar way, a tablet might be overkill for a 'home assistant', and the Amazon Echo seems designed to address that.
You answered your own question. Not every device used in a home (remote control, light switch, thermostat) needs to be replaced with a full tablet computer, even if it could be.