There seems to be a lot of variance in the percentage of police chases that involve stolen vehicles but the numbers seem to be in the range of 10% to 70% and even the low end of that isn't particularly low.
You also have the problem that if you steal a car and then run from the police the result is that they don't pursue you and send a ticket to the person you stole it from, that makes it a lot easier to steal a car, and then the percentage goes up.
Except it's almost never that. The answer is that people are stupid and impulsive.