Features of a rotting insect carcass
What you see after an insect dies is its chitinous exoskeleton. It is a tough biomaterial that takes time to decompose, given its structure, and few microbes can digest it, let alone as quickly as soft tissue. It's a little bit like human bones. The insect's inner soft parts will degrade quickly, just as with any dead, soft tissue. However, what remains looks intact, because the outer cuticle is resistant to deformation; it keeps its shape!
Exoskeleton decomposition by insects
Ants are known to, and can and will break exoskeletons into pieces and carry it away in parts. They are ubiquitous and exist everywhere, so it's no surprise you see them doing this job all over the place. Flies and worms and mites may also feed on the softer parts and on the yeast or microbes that flourish in the decomposing tissue. It is also possible that some insects lay their eggs into the carcass, which would in some cases be a rich source of nutrition for the young insects, when they are in their larval stages (what you may confuse for worms).
Poison deters exoskeleton decomposition by other insects
If you introduce poison into the mix, obviously the roach carcass with be contaminated and its exoskeleton will remain relatively less likely to be touched by other insects, since it remains noxious to them. The effect on micro-organisms would be less pronounced, but that depends entirely on the contents, concentration, volatility and weatherablity of the insecticide that was used.