No one said it wold be easy, and it's implicit any fundamental reforms are difficult, I'm just querying why people seem to laugh off the principle and wouldn't want to even try and work towards such a outcome?
Probably because all of recorded history in every part of the world indicates that it's impossible and counter-productive. It might look OK on the surface, but it's impossible to implement without making the problems worse rather than better.
What constitutes a lie? More specifically, who decides what constitutes a lie? Under your suggested system, whoever gets to make that decision can dismiss an elected politician by their own decree. Would it be a person? They would be very close to being a dictator. A committee? It would be like the House Un-American Committee during the McCarthyite period. Polls on "social" media? Very contempory, but probably even worse.
I'm not being flippant. Politics isn't always about things that are objectively true or false and known to be so at the time of speaking. Then, of course, there's bias. To some people everything Trump says is a lie, whatever it is. To some people everything Trump says is true, whatever it is.
I see your idea in much the same way I see communism - sounds like a good idea in theory but trying to implement it will make things worse.