What is the reason they do not train new staff?
Money, as simple as that.
Not only in ATC. For example, Spain's National Police Academy: normally around new 5.000 police officers per year. This year only around 150.
In my airport, there were 12 ladies doing the cleaning in the terminal building, 8 were fired on may (beginning of the high season). So now 4 ladies have to do the work of 12. In my TACC there were two ladies cleaning from monday to friday, one guy that did the glass/window cleaning, and one lady that cleaned on weekends. Both the guy that cleaned the windows and the lady that came on weekends have been fired. So now the two other ladies have to do their work also.