They do systematic checks for the winner, whoever has suspicious performance jumps one day, as well as random racers. They do take it very seriously and what you are saying is a bit far from the truth.
Some people will try, and some scandals will happen, sure. But it is not a widespread problem and is unlikely to become one.
Through axels have become a very convenient excuse for bike changes. Watching Cavendish have his wheels changed in one of the early stages this year was so slow. It took a guy with a drill like a minute plus to do front and rear wheels.
Some people will try, and some scandals will happen, sure. But it is not a widespread problem and is unlikely to become one.
See here, for example: https://velo.outsideonline.com/road/road-racing/tour-de-fran...