Why aren’t GP registrars asked if they want to do locums at their surgery?

I work in a big training practice and there’s at least 3 registrars soon to CCT, well-liked and been with the practice >12 months, clinically good, exams passed and work autonomously with little to no input from their supervising GP.

Wouldn’t it be cheaper to ask these registrars if they would like to cover vacant shifts than fork out for expensive post-CCT locums? Seems like a no brainer to me.