Simply because other parts of the transport industry use 2 - train drivers, for example. We should be at least as strict. The 6 sittings should also be 4. The reason for not doing one at a time is retention. You can't just learn and dump each subject because it all pops up again in other subjects.
I guess there is also an element of motivation (I'm sure you are BTW) - other people with kids and jobs do it in 6 months.