I always suspected this was partly because lots of sailors means a big navy base, and a big navy base means a booming local economy in that congressional district.
If that also helps politically that is a bonus.
Source: I once talked to some dude who was both important and in a Navy.
Most current sailors I talk to are overworked, training gets gundecked, following safety procedures and impede advancement - what's reworded is a "can do" approach that takes large risks.
I don't know anyone currently serving, in any command, who says they have too many people.
It's not so much that we were doing "unnecessary" work, but rather that many tasks which required a dedicated person were so trivial that any reasonably competent sailor could easily handle two or three of them at once, or the task could be even more reliably automated.
One quick example is the fathometer operator: When we were sailing in shallow or uncharted waters, a sailor had to spend hours doing absolutely nothing but watching the depth gauge so he could give a verbal warning if we were in danger of running aground. This is not a job that requires a person's full attention even in the most challenging undersea terrain.
To some extent it's necessary to break jobs down into simple repetitive tasks so that they can reliably be done in a high-stress environment, but the navy really took it to an extreme. The end result was the overwork and fatigue that others are referring to.
If you don’t believe a warship needs that many personnel on duty, that is different. Most of the crew aren’t there to sail. Cargo ships, for example, have a dozen crew at most. And their duty is to sail the ship from one port to another.
The Navy deliberately understaffs their vessels to meet unrealistic budgets. Overwork, fatigue, mistakes, and catastrophe are the inevitable consequence.
The only thing more predictable is the scapegoating of ship personnel and the untouchability of those who create the conditions which guarantee eventual disaster.