1. Halo 3 is not online only and even online games get pirated (It is possible to play a lot of MP games with pirated games on private servers.)
2. Count production costs, server maintenance and than sales become important to MS, they dont just need to break even on this, the game needs to keep generating revenue or it will cost them money.
3. Not everyone wants to drop a few hundreds bucks on a good pc and have the knowledge to keep their pc running correctly.
IF they ever bring Halo 3 to pc, the chance is that it will feature no MP at all, due to it being not only and older game, but also since on pc the have to maintain servers.
The chances are also very slim that it would be coming to steam, since Valve shares some of the profit made, making it harder for MS to even break even.
And of course is the animo there, if it's only the "hardcore" crowd that want's Halo 3 on pc they would never release it for pc, if its the general public, the chances are somewhat higher.