I'm not against a Decision for creating a custom OOB that follows certain construction rules. I also don't think that the current implementation is random. It can be a lot of effort to hand-code and maintain all those choices so, since you already roll your Planets and create Models using a procedure, it sounds like it was a deliberate choice.
From a gameplay perspective, it would make sense to create an OOB out of seven choices. However, how do you then model all the real world reasons that OOB's exist? A real army has a lot of administration, planning and training involved that makes it work.
There are a lot of compromises and very real world issues that just aren't going to be covered in a game at this scale. It also is the case that militaries develop doctrines, strategies and tactics which proscribe how soldiers should be organised. These organisations have to deal with people-centric issues like the IQ distribution of the recruits, the amount of time and money devoted to training, the strength and experience of the officers and non-commisioned officers who must made it all work as well as problems of class and social status (I remember a particular issue of certain modern day tank crews refusing to maintain their tanks because that is 'beneath' their social class).
I think the current system allows the developer, who knows all about these real world issues, to take care of it for the player. I say again, I am not against having customised OOBs. It doesn't bother me as a player of a 4X game. I just see that all the military sims have the idea of relatively fixed organisations for military forces and it is likely that there would be good reason for it.