They are installed in some cars. Drunks have actually recruited irresponsible and foolish people to blow into the apparatus for them.
Why can't Breathalyzers be installed in vehicles so they won't start if the would-be driver has a blood alcohol level over the legal limit?
Because real alcoholics will tear them out or find a sober person to blow in it
I had an employee once that had one in his car after a couple of D.U.I.'s .I wouldn't let him drink any after work but he did tell me that another person could sit in his car and blow in it and start it and he could drive. I needed him at work and that's the only reason he got that and not jail time but they can be tricked.
As others have pointed out ... They CAN be put in cars.
But any electronics can be bypassed if someone wants to do so. (Remember, cars ran just fine before they had computers.)
It is not installed in every vehicle because of the money it costs to do it. They are expensive.