So on one level you're right of course: MS aren't the only cloud provider in town.
However they're one of the big ones, and the only one who also has a gaming arm - unless you count Gaikai but that's not really apples to apples at this point.
On a technical point it's a lot more than just "spinning up a VM", there's a whole boat load of underlying infrastructure and SaaS (scaling processes) is very different to IaaS (scaling up and down VMs) and that's where some of the smarts are: how do you make that fast without overhead, how do you route traffic to the most geographically appropriate place, how do you auto-scale so and auto load-balance, how do you support the back-end data needs, how do you handle the vast quantities of data being thrown around - where do you store it to do your analytics, how do you slice and dice it, and on and on and on and on.
TL;DR: Cloud is more than just VMs.
So, yeah, it comes down a lot to software too - and MS already has a huge investment in this area, the cloud platform is incredibly mature.
So nothing stops MS being the only company to do this... but there aren't many that can do it at scale and there aren't any others that are doing it (again, except Gaikai which has some similarities but is a lot more 'IaaS').