Yet they are spending a crazy amount on hosting with Heroku when they could be doing it themselves and get cost savings plus know the entire stack.
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.
Also the amount of money really gives no indication of how optimized a site is or how much traffic it gets. I've seen sites that are in the top 500 spending ~3K/mo on hosting and sites that get 60K uniques a month spending 2x that.