Vst, Vsti And Mac, software access Options

[Mr. Jason E. Lec...]

i was just wondering somthing that may seem elementary to many of you. i love my mac and i love using ableton and reason on my mac. but some of my friends have some great synths/effects/instruments on pc that i just can't find a the same for mac. and there area lot more freeware/shareware plugins for pc as well. what i wonder is why most audio programs on mac can load vst, but some vst only work on pc? is there a converter that can make them work on mac so i can plug them in to ableton? i already know about virtual pc, but that's not what i'm talking about. and while im asking that does anyone know of a rtas to vst or au conversion? any suggestions would help.