Like most things, the truth is never pure and rarely simple.
Yes drivers are very expensive; the nature of franchise competition dictates that bidders will try to keep them to a level, although they understand that they will also be judged on service robustness, so cannot bid very low levels.
Most TOCs try to keep the numbers at what they think they need. TOCs generally want some rest day working, because not only would covering everything all the time be very expensive, but around 40% of the drivers rely on a regular supply of overtime.
Keeping the right numbers is however, harder than it may seem. Firstly turnover is hard to predict. You have to predict retirements (no longer a given at 65), poaching by other TOCs or simply the desire to move, ill health, safety incidents, internal transfers, promotions, even the number of trainees who will fail to pass out successfully.
There are also temporary shortfalls, both planned (training for new trains or changed routes) and unplanned (flu epidemics). Then there's incidents on the day, whether to the service itself or general incidents that affect staff (like a road being closed).
On top of this is human error. I have seen several instances where the calculations of required drivers contain mistakes; while it's not rocket science, the people carrying out these calculations are not rocket scientists.
So, temporary shortages are like death and taxes - inevitable. Nevertheless a number of franchises do have insufficient drivers. DfT recognise this and have recently commissioned a study to look into the matter.
