Legacy reason: Win XP 64bit.
it's most likely for the following reason:
deadline had to be met, not any small amount of time could be dedicated to set and test specific compilation tags for the PC version.
same reason the compiler optimisations where not checked.
since then, great modders proved to anyone that these simple tweaks could do miracles just by checking some boxes on the compiler.
now they had no more excuses to not include them and make the required minimal tests...
you have to understand that since without them the game still works (much worse but works nonetheless), in term of added financial benefits, it generate nothing but cost developper time.
15 years ago, gaming studio were mostly about creating great game and THEN get rich.
now, gaming companies are just another business, and the main goal is money. selling a game is just a means to an end, not the goal anymore.