This comment makes a lot of assumptions; the primary one being you are able to completely build your infrastructure from scratch. Which just isn't the case for the majority of companies or web sites out there.
It's not:
- because it's can't be done; - because we don't know the way to do it; - because it's better that way.
It's because it's very expensive. In your case, either because you don't create your whole system, or have a legacy system to use, doing it would be prohibitive. It's still a matter of cost.