Probably so that HMRC don't have to go chasing people who don't pay when they should.
There would be quite a few more chancers who'd run up a huge tax bill and then disappear I'm sure.
Just paid my first self-employed tax bill a couple of days ago after the accountant worked it all out for me, was a less than I expected, so quite happy really.
Definitely this. The people we bought our house from did exactly this, under pay tax and then emigrated to Australia. I seem to recall when I opened up a letter from HMRC (who are really bad at acknowledging a no longer lives here return) they had a tax bill of about 180k!
I know they have parents that live in the UK, so no idea if they ever plan to return but I'm firmly of the opinion that when their parents die the taxman should get his chunk of unpaid income tax before they're even allowed to see it.