When you get millions in funding its easy to piss away on the highest tier services. When you start from the ground up, every cent is managed.
Unless going custom allows you to do something that you couldn't otherwise do (cases which are rare, but existent), it's just optimizing margin. And you can't margin your way to success. (To failure yes, to success no.)
As noted above, their hosting was a big number, but it wasn't a dominant cost. It's like Amdahl's law of money, it doesn't matter how little you spend in one area if that area isn't dominating your costs.
And all else being equal, it's usually the wrong choice; the best way to grow the bottom line is to grow the top line. If you can spend X weeks cutting costs by Y$, or X weeks increasing revenue by Y$, grow the top line. It's fuel, it gives you options.
The thing I miss the most about doing it our selves was the raw fun of it, and how efficient it felt. But in retrospect it was a lot of time spent on things that ultimately didn't change the outcome.
As someone who has worked in this space, I can tell you that the skills simply do not exist. They don't hire for them, because they all ("all") use outsourced hosting and deployment frameworks that insulate them from these things.
Plus, you don't have that all-important "Cloud" buzzword for them to throw around!
I'm 20 myself, and while I'm not the hottest shit around, I have been working at a bank for the past two years. I've been exposed to some really smart people with extremely challenging problems, way more so than creating a pretty service in a few weeks and getting crazy amounts of funding for a business that's most likely going to fail because we're just that, kids.
Before coming into my role, I truly did think I was the best because I was able to create pretty websites with whatever is the latest JS library everyone is fawning over. That illusion was destroyed immediately when you start designing and working on infrastructures that have to handle millions of payments a minute, working with multiple vendors, business units, testing units, and various other departments you need to familiarize yourself with.