The unnamed protagonist of Doom 3 is probably actually called "Marine Transfer", considering that is what appears on the passenger list at the start of the game (alongside Elliott Swann and John Campbell).
Classic Doom, the Arachnotron and Spider Mastermind have, compared to other monsters of similar strength, a disproportionately high chance of flinching when they're hit and being interrupted from shooting back. They're giant brains on mechanical legs. Of course they're going to be less resistant to pain.
Which has a bit of fridge logic in itself, since a brain is the ONLY part of a body that cannot feel pain at all (it processes pain but actual harm done to the brain itself can't be felt)
Possibly it's not actually pain, but unintended spasms caused by the punishment being rained upon them, then?
The final boss of Doom II is John Romero's head on a stick obscured by a giant image of a Satanic monster called the "Icon of Sin". Appropriately, it is the source of all the monsters you've been fighting up to now, which of course is John Romero's duty as a game designer.