Entrenched interests mostly pretend to want to do the right thing or lie outright about wanting to do the right thing.
When elected, MPs and Parties linger too long for the average person, who sour on them, and they're booted out in the next election in favour of the other side.
The people who would probably do the right thing never quite manage to get enough votes to actually do so, except now and again they can use their influence to improve things a bit for everyone.