You don't even need to modify much. the games do run. the sims 1 needs to be run in windowed mode because it has no concept of the resolutions we play at these days. And 2 just needs a minor compatibilty patch to run on systems who no longer support DX9. 3 is just a product of it's time and would take a lot of effort of a team who has no clue how the game was made (remember, old maxis is gone) to completely rewrite it to take advantage of modern systems.
Well that and you'd basically break every mod in existance and might *also* have to rework all the store content.
