Because Sony can choose other company easly  like Amazon or google. So My point is still correct . Consumer is a king. You got the money you got the service.  

If I were to take a guess, maybe MS is more useful to Sony. Sometimes we stories where MS can be surprisingly hands on or maybe even share some tech. By that I mean MS could help make Sony's online services better.

Maybe MS is also trying to make things happen with Sony to somehow expand their userbase. For example MS was trying to get Xbox Live on PlayStation. Hence, MS does have their own interests outside of just letting Sony use their infrastructure.

While there could be many reasons, the most obvious and clear reason is explained in the post below...

The other part of the answer could be summed up by only777's signature which says, Sony wants to make money by selling art, Nin wants to make money by selling fun, and MS wants to make money...

Keep your business friends close, but keep your business enemies closer, because who knows what they could be up to? If XB is using the MS cloud...

MS Money Plus Premium... PS Plus... Rumors of a potential PS Plus Premium like service next gen... PS + MS cloud...

Yes and No..

Yes) Sony can choose another company easily.. But would it be a better deal than what they have with MS? probably not, would the services be suited for what Sony needs? probably not, would that Cloud be better than Azure? probably not.

No) It seems Sony went with MS based on a better deal and coverage than the others. Will wait and see why soon enough. MS doesn't need Sony, this is Sony needing a Cloud service that benefits there gaming division. MS would have continued on there merry way while Sony will struggle to compete against mega corps like Google, Apple and MS in this department.

So your point is not correct. This is a corporation deal between two giants. Lets just be grateful that two power house companies are working together to help improve our gaming industry.

Sony wouldn't struggle, because they clearly have decided to outsource their cloud services. Perhaps they have been thinking about building up their Gaikai thing, but came to the conclusion that maybe partnering would be more efficient. This way it is also easier to pull out, if the whole game streaming thing doesn't pan out like intended.

If it wouldn't be MS, it would be someone else. It's not about needing someone, it's just business for both of them and partnering with MS I think is the best option for Sony because they have practically been doing the same thing for years and they know the gaming scene the best.

Nobody knows either it will be good for Sony or Microsoft,  because nobody able to predict the outcome for this project for the future. Would they have better deal with Microsoft instead with others ???  probably not.  It's a not a long term plan. The deal can be change anytime. Also Cloud gaming need more than 30 years to become mainstream. Sony can still find more partner , it's just like using Taxi. You can use Uber or u can use other service. Sony not in the hurry or the one in need for the server.

I wouldn't say Cloud gaming needs another 30 years more, these companies want to push it now hence this megaton of a deal that was just made.

Anyway let us be grateful that too major companies are pushing hard to help push our industry forward.

The company can push as hard as they want but the reality is totally different . Infrastructure, government policy, politic ,  and other problem and conflict of Interest  Stop the progression of technology and gaming.  The Internet infrastructure is really bad and not in balance (The need of high speed internet are not equal to the progress of infrastructure, even in first world country). 

Cloud gaming need, Speed, latency , Price and content. Even if all company joint together this problem will still exist. Also the console Industry are in the golden era, only on the losing side hate to see a console and local hardware device exist. 

Cloud is the future, I am agree . But for now it's still premature and infancy. These company are preparing for the future but they are not in hurry.  So I am glad they joint in hand for the benefit of both, gamer and world progress , but it need time. 

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You know Streaming works right now with PS Now and Stadia right? and however long Steam and mobile gaming had offered it before. Streaming works on average networks. Lets see how good xCloud is at offering a Streaming option because they promise barely any lag, and with 5g due out soon, net speeds wont be an issue ever again.

I dont care for Streaming as i much prefer the traditional way however you cannot denie this is the direction corps want to take because its where the real money is.

So is Nintendo now, lol