SDKs are not that hard to make. sometimes they are just lite versions of the professional tools developers use, which means all they have to do is apply a couple scripts, other times they are just executables coded to decompress and unpack the game files into workable junk. both those things can be done by someone else with time and a little knowledge, and a lot of people can do that. and then they can share freely.
whatever, if people managed to take the assets of AvP2010, then they sure ass hell are going to work just fine without an SDK. it's just a matter of tipping hat to the community and actually wanting it to grow and extend itself further. however, this means they can't release outrageously poor DLC or churm out a sequel that's barely any improvement, so that's why you don't see games with them nowadays.