TL;DR - They've said repeatedly that they would rather work on their product/projects than have to deal with all of the details themselves.

The crazy amount they're spending should give you an idea of how much throughput they have and what would be required for their own setup. That means a lot of design, additional time implementing, more time updating to new tech (including on the software side, in gems, etc), then in the end, ongoing maintenance.

There's probably a lot more that I'm not mentioning as hey, like them, I'm not interested in spending my time developing, purchasing and administrating my own hosting systems, either. I have apps to write.