Because fines are automatic. If you register a vehicle to your name and don't insure it within 28 days you get a €500 fine. The government can fine you up to three times a year for a total of €1500. And that is if you actually pay the fines. If you do not pay the fines and let the fees stack up, you are looking at around €4500 per year.

And if you are caught driving uninsured that is a €700 fine on top of all that. With many police cars now having ANPR systems it just isn't possible to drive around uninsured without receiving fines that cost way more than just getting insurance.