"There's a big strategic risk in you guys offering web hooks: it facilitates competitors piggybacking on your event detection and building high value response services that compete directly with your own. In other words, you become a piece of costly-to-maintain middleware and others capture the real consumer value." -- https://news.ycombinator.com/item?id=4344335

In other words, something like webhooks hurts the bottom line of this "open platform" (quote from OP.) Maybe IFTTT is only useful because a lot of the services it consumes don't make it easy to do these things, because it would diminish their own value. It's greedy walled gardens all the way down.