It's true many stay. However, the plan with these two is to progress them to starters. In Kopech's case he just needs to build innings because he hasn't pitched competitively in 2 years. He will be a starter eventually. In Crochet's case, no one knows if he can handle a starters load so he needs to slowly build up and see what happens.
In both cases, neither could be a full time starter this year. So the choices were to be bullpen guys in the minors or bullpen guys in the majors. I personally would not like the option of having them be starters in the minors and shut them down really early in the season. The advantages of the MLB are they continue to learn how to out MLB hitters and help the MLB team. I