Privatisation certainly hasn't helped the driver issue. Where you would have had one big BR depot at one location, with drivers starting at the bottom and moving up the links to get different, better work content, and more pay, you now have 2, 3 or more different depots at the same location. Natural progression isn't as simple. You can't just move upwards, you have to apply for a vacancy when it arises and go through the whole recruitment process. With different companies come different working arrangements, different pay negotiations and different budgets depending on the work.
Take Manchester Piccadilly for example. You have Northern, TPE, CrossCountry and Virgin driver's depots. All paying different rates of pay, with different terms and conditions, different work content etc etc etc. The costs of training drivers falls on Northern and TPE mainly, as they tend to take in all the new recruits. People often leave Northern to go to TPE, and TPE to go to XC and Virgin. That is exactly how it worked under BR with progression and promotion, but now you are changing companies at the same time.
Also bear in mind that when a driver gets a new job with a new TOC, he may have to work only 3 or 4 months notice at the TOC he currently works at. But it will take that TOC 12 - 18 months to recruit and train a replacement, unless they too pinch a qualified driver from somewhere else. So for possibly over 1 year you have a shortfall while a replacement driver is sought.
As you have no idea how many driver's will leave each year, how many do you recruit? Recruit too many and you risk them all being sat around with nothing to do in 18 months time, and then you have higher costs than expected, and risk being criticised by the press (should they find out) for being wasteful with all the hard working commuter's money.
You also have no idea how many will have incidents, be taken sick, be restricted for one reason or another for example.
Also, at some smaller depots for some TOCs you can have a very high % spare coverage, but still not be able to cover the jobs. Think of this.
A small depot has 8 duties per day, 4 early shifts and 4 late shifts. If you have 100% spare cover you would have 4 rostered to each early shift, 4 rostered to each late shift, and 4 early spares and 4 late spares. Lets say of those 8 drivers rostered to work in the afternoon:
1 is on allocated annual leave by rotation.
1 is on daily leave he applied for 9 months ago.
1 is sick.
1 is a vacancy due to somebody yesterday having worked their final day after having worked their 3 months notice before leaving to go to another company, and his replacement is still in training.
1 has been restricted as he had an incident / irregularity last week which is still being investigated.
The other 3 drivers are available to work.
But you have 4 duties to cover.
How do you cover the remaining duty?
In the morning you have all 4 duties covered and 4 spares sat there doing nothing. But none of those can cover the uncovered late duty due to the limit of movement from their base spare turn and their duties the following days etc.
A perfectly possible scenario even with a wasteful 100% spare coverage.
As it happens TPE has a higher than industry average spare coverage, which ASLEF itself has said.