Call me cynical. but I don't think that there are many politicians who join a party out of conviction these days.
They join the party that they think will get them elected and in a position to do themselves the most most good.

Labour are just as dishonest and self-serving as the Tories and the honest members of both parties could be counted on the fingers of two hands.
Then they wonder why no-one bothers to vote.

Put an 'abstention' option on the ballot paper if you really want to get a feel for what the public thinks.