Hi,
I have also been asking myself the same questions for a little while now. My reasoning comes to these two answers:
1) Heavy traffic jams! After carrying out a heist, even the police do not need to chase after you. They can just follow your run-away car while walking very briskly and catch all of you with minimal effort,
2) Members of the police force do not have too much patience with armed robbers as they used to. Quite a number of years back in the 70s and 80s, the likes of John Kiriamiti and Kimami Kihika if I remember correctly, used to play cat-and-mouse games with the women and men in blue. Nowadays that is a no-no.
Martin.