It would simply be easier to do it yourself sometimes. Art Markman makes a valid case for involving other team members to teach them how instead of assigning them tasks. He suggests managers need to stop thinking of passing off responsibilities as delegating. Period. Delegating with this mindset sets your employees up for failure.
Structure experiences so that your employees are able to work their way up to a challenging task.
Source: Harvard Business Review, October 18, 2018. Link.
When you assign someone a task for the first time — with no prior training — simply because you are unavailable to do it, their chances of succeeding are slim.