A few reasons
1. Valve is a competitor on the PC market. They don't want to endorse the competition because they're still trying to break into it even though its a hopeless cause right now
2. Microsoft is keeping the lid on the XBOX360's so-called cookie jar pretty tight. They're big on console security and they don't want third party software possibly opening a can of worms the size of the PS3's system compromise
3. (Toward's the TF2 comment) It costs Microsoft money to roll out updates. The reason you're paying for Live isn't for the multiplayer, paying to get multiplayer is just a way to entice people to pay. You're paying for the rest of the service, especially content servers. This becomes apparent when you look at games like Super Street Fighter IV, where Capcom released costume packs that were not included on-disc, but people who didn't buy them can see the costumes when they fight somebody who did on XBOX360. On PS3 people with the costumes show up to the other person as a stock costume. Microsoft can afford to do things like this because they charge for what's (pretty undeniably) a superior service. Whether or not Live is a superior value to the consumer compared to PSN is up for debate, but ounce for ounce Live is a stronger service, and you can see where the money is going when you look at details.
Also I don't think PS3 is getting Steam, PS3 is just getting Steamworks. All that means is PS3 players can trudge along behind PC players on Portal 2 coop which got somewhat console-ized to accommodate the PS3 crowd. While this is pretty cool, its not necessarily a system seller, and I don't like how it changed Portal 2 between E3 2010 and the final release.
Just my two cents
