Assigning work to someone is really assigning the responsibility of doing the work. You’re saying “get this done”, not "do this".
If they don’t know or can’t or don’t have the time, or know someone who will do it better, let them delegate the work but make sure they understand that the responsibility for delivering is still theirs.
It’s empowering and motivating for them, it frees them for other tasks they are better at, it enables them to try their hand at delegating and managing and it gives you the opportunity to see how well they do it.
As long as the outcome is the one you want, how they get there is not your problem.