More and more APIs use HTTP as the transport for various reasons, but one of them is that HTTP has considered large number of edge cases and has been tested "in the fire" - rolling your own custom API with security and encryption is much more fraught with dangers.
But I would bet that those only use the GET or POST functionality of HTTP, and that the real reason it's used is because it's easy to quickly get something up an running.
It's a short-term gain, but quite inefficient in the long run.
ZeroMQ is a perfectly valid replacement for HTTP in internal networks. There's no reason for everything to be human readable.
