Different dating apps cater to different mindsets towards dating. Some ban pictures in favor of words; while others only work if you have great pictures; then others want one to mention their annual income ranges.
Strangers don't belong in that model because they are not friends. Most people want to keep strangers away from your real life until they become friends so having a third party site keeps that experience away from preying eyes. Once a couple is formed they will connect over facebook and start sharing their experiences.
You might be right though, it might consolidate in the future if people figure out really good interfaces that make it work well for many use cases.