Facial Recognition Services differences

hello! I hope someone could give me some insight on this: why are there so many Face Recognition Service vendors out there? A quick google search gives me a list of at least 15 vendors (some are from a well-known company like Microsoft, McAfee, Amazon, etc.) and some business even build their own in-house solution. How different from each other are they? Which one should I use or should I build my own? Thanks in advance!