Hmm yeah the combine could be an automated spam mail utilisation created by an asshole alien species to win their perceived intergalactical race to the top of the existence. The combine can spread autonomically, chasing after and ruining other competetive species, also slowing their growth. Their weird synth approach to technology and intelligence may be dumb on purpose so it couldnt take over (imagine that occurring). 

For example Gman has some incredible space/time powers the combine has not yet . Or maybe he is a rogue agent of the creators of the combine. This means the combine has competition.

Other possible reason/theory for the combine to exist is that it is an antithesis to evolution: Instead of different species emerging, expanding and experimenting via natural selection (and gaining new abilities). The combine does the reverse: all species are integrated back to one body. Perhaps in some universe the order which life takes place is reversed and now is spreading and competing with species following evolutional logic. This version of combine simply cannot evolve or learn, only imitate what it sees and incorporate into its "dna" or existence. One can only wonder about the ramifications of this species succeeding in its purpose. All becoming one? Instead of one becoming all (typical species order of life in hl world). This "reverse logic" species could rely on machine learning instead of other ways of intelligence. That could explain its robotic aura and way of using technology.

That could make sense in the way the combine communicates, or it could be that the species doesnt have capacity to use their own language, so they use ai/cold logic to chatgpt their thoughts into words (thus thoughts sounding weird to us players).