You can't turn contingency staff out willy-nilly. There is a substantial course on PTS/rules/route/traction to be delivered and there is a limit on the number that can be trained each time (low single digits), all within the capacity of existing training school schedule utilising trainers who also have to work with new starters in regular positions. All of those signed up to the course will also need their work covering for release on all those days which isn't a luxury many functions can afford these days at most TOCs.
People come and go at all TOCs. You can only work with what you have. Some contingency staff have booked holidays, attendance at external events, appointments with business partners, etc, long before the strikes were announced, which cannot always be cancelled, nor should they. If additional staff become available on the day, more services will run, but delivering what is advertised is of utmost importance as there is likely to be a severe lack of capacity to start with so not overstretching your planned timetable is rather important otherwise cancellations will likely have much bigger impact on people's journeys compared to a normal day.
