Why does it blow your mind? Various obvious and sane reasons for this, including cost. I bet many of those companies you have in mind “buy” Windows and macOS, and if they are sufficiently big, most certainly buy Oracle or SAP for their corporate operations, finances, and accounting. It’s usually only the production side of sufficiently large internet-scale companies that is biased towards building. Most of the time you can easily explain it with “it’s cheaper than the contract with supplier”. Often, it is strategic ownership over your fate and not being locked into the vendor that comes into play as well. The vendor positioning in the market and its leverage may also change over time and pose a risk down the road (getting acquired, changing focus, abandoning the product, going out of business, etc.) Many times it is just that their problem is so unique to their scale that the generic solution does not technically work for them or the pricing model is not designed to be a fit.
In the particular case of nginx, I can tell you their reputation is not great in adapting to the users’ needs.
In the particular case of nginx, I can tell you their reputation is not great in adapting to the users’ needs.