The general idea is to have lots of infantry that cant die and can kill everything. Introducing anything larger, except for hazardous environment combat such as in space, is not prudent. The selecting of a mobile infantry unit that can overcome all and any reasonable obstacle in their paths, the equiping of that unit and the training is the most difficult thing any military can do, and if the most rewarding. Why introduce tanks when you can have one soldier carry this around on his back and act as a heavy weapons specialist? Personally I believe well trained and equipped infantry can overcome any obstacle but impassable terrain (you can't swim the atlantic too easily without help), and as such should be the unit type of choice for any sufficiently advanced military.
The Lamia power armors, and power armors in general, are extensions of this. You do not need large, clunky mecha and tanks when you have infantry sized starship killers. They're more mobile, can be built faster, are more diverse and can handle nigh on any problem thrown at them, from all out war to simple peacekeeping or heavy lifting.
THIS is where occams razor applies. Not the equipping of such troops, but the type of troops to be equipped.