Here's the actual reality: We don't know what most of these people are doing, especially during jump. We don't know how much automation is going on. Clearly a lot, since they don't even need enough crew for proper watches.
Quartermasters in the US Navy (the navigators) manage to find ways to be busy 40 hours a week. No one is making a 30 minute task check every two weeks and getting paid for it, even if that's all we are told the Astrogator does.
There is vastly more work involved in being a ship crew than the game bothers to enumerate. Maintenance, continuing education, watch standing, lots of other things.
As far as the rules go, I don't think there is actually any penalty for not having those positions or having one person fill 8 of them. In CT, not having all your engineers increased the mishap/misjump chance. In MgT2e, I am not aware of anything bad happening if you don't have both Engineers your ship "requires". Because we don't bother to define the actual work they are doing, because it isn't relevant to the gameplay.
Are those crew requirements a legal requirement? You need this many people with this level of certification to operate the ship in accordance with safety laws? Are they are workload requirement? If you don't have that many people, stuff won't get done? We don't know. The lack of mechanical penalties could be taken to mean the former, but it probably just means no one wasted time on the topic.