SPOILERS AHEAD: While the second answer posted above could be correct, I suspect it's not. As a fan of the show, and ONLY a fan, I've nonetheless paid very close attention to all aspects of it (and have consumed just about all sci-fi shows and movies going back to "Lost in Space" and the original "Star Trek" in the 60s).
As has been shown time and again in the show -- including in the most recent episode (Season 4, Aug. 10 -- Episode number isn't available to me at the moment), although many parts of Eshpheni technology are solid metal, and thus totally mechanical, many others are of a biomechanical nature, implying that the Eshpheni (whether they're totally biological beings, or not), have used integrated biomechanical features in almost all of their technology.
As was in evidence when Tom reached inside the "squishy portal" to activate the hatch to get inside the crashed Eshpheni ship, "something bit him," in other words, there was something "biomechanical" inside that grabbed his hand and decided based on it being biological to open the hatch. Then, when he happened to touch one of the orange, glowing, gelatinous-looking panels on the inside of the ship, he was not only badly shocked, but "something" came off that panel and imbedded itself inside his arm, and began writhing under the skin like it was trying to work its way up to his nervous system. Dyangann had to quickly wrap a TIGHT belt around his upper arm and then YANK the thing out by the roots, and when he did, it was like a hard shaft with little wiggling worms on the end of it -- OBVIOUSLY biomechanical...
The same was true when Anne transplanted three of Ben's "spikes" to Maggie's spine to try to heal her paralysis. As she removed each one, the end of it wriggled and jiggled like the tentacles of a live octopus pulled out of the water, and as soon as she gently placed it on Maggie's back, over her spine, it "knew" exactly what to do, and burrowed right down into her spine.
Although the "mechs" could have some sort of "live" pilots inside them, I think it's highly more likely that they're more of the same sort of biomechanical weaponry, controlled by the minds of the Eshpheni, and that when any sort of "blood" oozes out of them when one of them is "killed" it's some of that biomechanical "life" that's draining away, not any actual live being inside of the "mechs."
In the case of the "skitters," they were once independent beings just as are/were humans, but were conquered by the Eshpheni and were put under their control the same way the Eshpheni are now turning humans into their slaves. We have no way of knowing if the "skitters" originally looked anything like they do. They may have walked on two legs, as well, but changed drastically as a result of the Eshpheni "evolution" process. We know at least ONE had the same control device inside it, tied to its nervous system that once was tied to Ben's spikes, controlling him. We don't know if they all do, or if many of them either willingly gave up and became slaves in order to live, or were "converted" into what they are now by a process similar to what Lexi showed to Ben on the Aug. 10 episode. But we do know that THEY are biological beings, even if they do have some technology implanted in them.
At any rate, the "mechs" were obviously designed to move like the overlords. Although humans have designed some combat robots to disarm bombs and some flying robots to take pictures or drop bombs, there seems to be an overwhelming desire to create robots that walk and look similar to us. There's no reason to believe another intelligent life form wouldn't see things in a similar manner, even though bipedal movement has some major disadvantages as far as balance and swift movement and maneuverability go.