I think another big reason wasn't given, Microsoft has faced various suites over the last decade about being a monopoly. If Microsoft purchased the second/first largest games publishers I'm sure someone (Their competition or government) would push for the government to intervene and prevent the sale.
Why did Microsoft close Ensemble and allow other studios to leave or merge with other companies. Simple Ensemble created only one franchise (AgeOfEmpires) and were good at only one genre (Real Time Strategy). Microsoft milked them for every last dollar by allowing them to finish HaloWars before terminating the studio.
So why doesn't Microsoft purchase smaller studios? I really don't know. Why doesn't Nintendo or Sony for that matter. It probubly has something to do with the gamble. Every time you aquire a studio you are taking a major gamble. First your gambling on their talent, if the studio isn't that great then buying them up might not be a good idea. Secondly your gambling on their IP's, notice when Nintendo took over Retro suddenly all their IP's were shelved and the studio was assigned Metroid. Thirdly your taking a gamble on the market, will the world be ready to buy these games and so fourth.
Now you can say, well why doesn't Microsoft, Nintendo and Sony buy more established studios and publishers? Their are alot of complications to buying a bigger studio. Look at what happened to Rare, Microsoft bought the studio and the staff loyal to Nintendo bailed. Then Microsoft began assigning some new projects and more of the studio bailed. Today Rare is not the same Rare that Nintendo owned, all that really remains are the IP's which Microsoft has been unnsuccsful at capitalizing on.
Then lets also remember bigger studios and publishers need to agree to a merge. If the studio/publishers board of directors don't agree on selling then a forced take over is required. In order for a forced take over to work 51% of the shareholders need to agree to sell their shares. Take Rare as another example. Microsoft tried to buy it from Nintendo outright. But Nintendo chose not to sell. Nintendo owned roughly 49% of Rare and their decision and the boards decision meant Microsoft had to buy the studio from its investors (Stamper Brothers...etc...etc..) this lead to a ballooning cost of over 200-million. Once Microsoft owned controlling shares of Rare Nintendo had little to no choice but to liquadate its remaining percentage.
But do first party publishers have hundreds of millions to blow forcefully taking over bigger studios? I remember last year EA tried to take over UbiSoft. UbiSoft took notice and urged share holders to cease selling out. UbiSoft fought off EA, sure EA got tons of shares but they were unable to rest control of UbiSoft.
Would it be in Microsoft/Nintendo/Sony's best interest to sieze assets? For that 200+ Million what has Rare brought in for Microsoft? Nothing close to the sales and success it had with Nintendo. If EA successfully took over UbiSoft (Which some estimates said would cost over a billion). Would EA really make its money back on UbiSoft or would they loose alot of dedicated staff studios and IP's leaving them with just UbiSoft's owned catalog?
Is it really worth the gamble for any of the bigger publishers to take one another over?