It's really not as simple as recompiling with 64bit. There are likely numerous assumptions of a 32 bit system all over the codebase that would take weeks of eng

It's really not as simple as recompiling with 64bit. There are likely numerous assumptions of a 32 bit system all over the codebase that would take weeks of engineer time to attempt to understand the scope of let alone fix. Casting pointers through int, tightly bit packed structs or a number of other little things. The work has been done on newer versions of source sure, but that leaves you with two bad options. Spend lots of man hours back porting to a dead engine branch for every single game or port those old games on to new engine versions and spend even more man hours trying to update incompatible bits and hope you haven't introduced new bugs everywhere. Not even close to worth the time for a tiny percentage of gamers for games that were released 10+ years ago.