Congratulations on completely missing the point and issue at hand.
If you care to do a little research you'll find that in many situations they do not.
And I don't just mean delaying services. Many drivers are rostered breaks in locations with no toilets and no facilities apart from the train they are on.
That is without getting into the issue of drivers who delay trains to use the toilet too often being seen as troublemakers or militant.
Until the matter above is resolved properly - CCTV in cabs is a non starter
I can genuinely see two sides to this.
Drivers are well-paid professionals and there should be a level of mutual trust between management, the safety authorities, the passenger and the driver, such that there shouldn’t need to be grounds to be wanting to look at what was going on inside a cab.
Likewise, we have seen only one fatality in over 25 years that has been caused by a driver error, although this increases if one includes the Croydon tram derailment (there’s then a debate to be had as to whether that would have happened had the track layout and safety systems mitigated against a risk that was probably fairly forseeable). This isn’t massive justification for an extensive programme of retrofitting in-cab cameras.
However there is a flip side to this coin, clearly there have been some incidents where things have been going on that shouldn’t have been. Again matter for debate as to whether this is the thin end of a wedge, or whether they have led to incidents precisely because they are isolated cases.
There’s no right or wrong answer to this. I wouldn’t want someone watching me scratch my ball, but equally I don’t want to be on a train where the driver may be on the phone. If we end up with a really serious incident where cab distraction is a factor then the balance of argument is going to tip.