There reaches a point where leadership requires you to take the manager gloves off and get your hands dirty. For small teams or small problems, a good leader/manager can be both one and the same.

However as the size of the team or problems grows, it becomes more difficult (think n^2) to tackle both issues at the same time with the same person.

Management issues (firing, hiring, expenses, budgets) and Leadership issues (decisions, progress, advocacy, examples) on a team of 5 may be doable by one person, but may be stretched with one person servicing a team of 15 or 150.

Or if there is a ton more problems in one area vs. others, the challenge of dealing with that problematic area can leave a gap on the "less problematic" areas, which eventually turn them into actual problems. That's my guess as to what people complain about the most.