It's the exact same reason why most companies don't just run their own power stations, and instead buy it from a power company.
Computation has become a utility these days - this includes the fat ISP lines and connectivity etc, not just the CPU and harddrives. These things have economies of scale that smaller companies cannot truly reach, and will pay a huge fixed cost if they want state of the art management, monitoring and redundancy. So unless you are a massive consumer, just like power stations, you really don't need nor want to build your own.
Computation has become a utility these days - this includes the fat ISP lines and connectivity etc, not just the CPU and harddrives. These things have economies of scale that smaller companies cannot truly reach, and will pay a huge fixed cost if they want state of the art management, monitoring and redundancy. So unless you are a massive consumer, just like power stations, you really don't need nor want to build your own.