Looking like terminators with their we'll be back reanimation was the point originally.They kind of remind me of Terminator T-800's. The wiki says they are alien humanoid skeletal robots. If they are aliens and robots, it makes me wonder who built them and why did they use a humanoid configuration?
I don't know much about the lore other than basic stuff. I just play the games.
Now, it's the biofurnaces that did it and most things in 40k(the game) are humanoid anyway(orks, eldar, tau, votann, a large number of chaos daemons, etc.), and the lore(this is covered in infinite and divine) is that apparently their memories may have been fucked with in the biofurnaces(there was a c'tan referred to as the deceiver afterall) or just due to entropy over time ruining their memory, they don't know what they previously looked like and no examples exist so necron characters(the necrodermis is still basically liquid metal) just defaults to whatever their equivalent of a brain thinks they should look like.