The government don't give a monkeys. Immigration that drives down wages makes services more affordable so more people can pay for them so the economy is boosted, which reflects very well on the government. The driving down in wages has a huge effect on the individuals affected though - people are losing work, having to change long held careers or turning to welfare to cope. This problem on a national or even constituency level is tiny, but on a personal level can be absolutely huge.
It might affect MPs if their home towns were becoming overcrowded, they were losing work, they were struggling to afford housing or their kids were in classes far larger than they should be. Fortunately for them, they have wages and expenses that allow them nice houses in nice areas, jobs that are safe for 5 years and they can send their kids to private schools.
Lots more people would vote for UKIP if they campaigned as strongly on the sovereignty of the UK as they do immigration.