Well, that actually happens a lot, with many causes.

For example: manager is perfectionist (at some level), it takes more time to delegate than doing it yourself, there's some attitude from the subordinate to undermine the manager, there's lot of work in a specific season and manager prefer to do it himself than risking his job due to a bad work form a subordinate

In an ideal world, of course the manager would replace the subordinate. But in real life, sometimes you can't find someone in the adequate time and is better to have him doing 75% of the work instead of 0%...