Yeah, the never owned the Oddworld devs or Bizarre. 

Former studios 

• ACES Game Studio: Microsoft Flight Simulator series, Combat Flight Simulator series and Microsoft Train Simulator series. Closed on January 23, 2009 in a process of ongoing job cuts due to Financial crisis of 2007–2010.[9][10][11]
  Ensemble Studios: Age of Empires series, Age of Mythology, Halo Wars. Disbanded on January 29, 2009.
  Carbonated Games: Developed for MSN Games and Windows Live Messenger. Disbanded on March 27, 2008.
  Digital Anvil: Brute Force, Freelancer. Disbanded on January 31, 2006.
  Hired Gun: Halo 2 for Windows Vista. Disbanded in October 2007.
  FASA Studio: MechWarrior series. Disbanded on September 12, 2007.
  Indie Built: Amped series, Links series, Top Spin series. Sold off to Take-Two Interactive in October 2004, now disbanded.
• ACES Game Studio: Microsoft Flight Simulator series, Combat Flight Simulator series and Microsoft Train Simulator series. Closed on January 23, 2009 in a process of ongoing job cuts due to Financial crisis of 2007–2010.
• Ensemble Studios: Age of Empires series, Age of Mythology, Halo Wars. Disbanded on January 29, 2009.
• Carbonated Games: Developed for MSN Games and Windows Live Messenger. Disbanded on March 27, 2008.
• Digital Anvil: Brute Force, Freelancer. Disbanded on January 31, 2006.
• Hired Gun: Halo 2 for Windows Vista. Disbanded in October 2007.
• FASA Studio: MechWarrior series. Disbanded on September 12, 2007.
• Indie Built: Amped series, Links series, Top Spin series. Sold off to Take-Two Interactive in October 2004, now disbanded.

Because Microsofts real enemies are Apple and Google. They are growing incredibly powerful and are a real threat to Microsoft. Sony is like a far far far infinitely far distant third.

Because they would prefer to spend that money instead of ensuring 3rd party support by paying to get timed exclusives, exclusives or exclusive DLC. There was a press release a while back where MS reps stated this, but I can't be bothered to find it.

I have no idea man. Microsoft's first party is very weak IMO. Like besides Turn 10 and Lionhead they really don't have much.

If I was MS I'd try to buy way more studios like Rockstar, Infinity Ward, Insomniac, etc.

Because they don't need to.

They have an awesome relationship with a lot of 3rd party developers. They look at promising games and publish them. That is what made games like Mass Effect, Gears of War (2 / 3), Crackdown (2), Alan Wake and Infinite Undiscovery exclusives. It worked out.

Milk Halo MS gogo!

I rather prefer them to stay that way, throwing money to 3rd parties to develop 'timed exclusives' that will be cheaper and better when the PS3 version come out.

They want to make 3rd party developers happy, and why buy a developer if they already make games for your console. It's less expensive to invest in 3rd party than to buy a studio.

In my personal opinion, Microsofts lack of 1st party devs is their biggest weekness.
Both of their competitors put out many many great 1st party games, and most Microsoft "exclusives" become multiplat to PS3 down the line.

Just imagine if MS bought EA or Acti-Blizzard. It would be game over!
even if they could pull devs like bethesda or ubisoft away and make them 1st party, it would be incredibly valuable.
I like the idea of the exclusive though.

I wish every game was exclusive.

MS didn't disbanded studios like Ensemble - they made restructurisation disbanding studios but at the same time absorbing key development teams of those studios into 343 Industries and other studios. it's easier and cheaper to oversee small development teams in one place by one executive than having 10+ different studios each with few development teams, 10+ executives/producers overseeing their work, 10+ R&D teams to support them, MS supervising guys in each studio etc.

plus they don't have to maintain those studios when they don't develop any games unlike sony.

MS owns IPs and few bigger studios making their most important franchises and couple studios on second party deals - it's cheaper and without owning original developer of each IP they can still make a deal with him to make new entry in that IP just like Mechwarrior deal with FASA or whole point of making Feelplus studio by Microsoft(yeah it's independent development team but they still have to work for both mistwalker making 360 games and microsoft).