Why is it so complex to convert a video file into FLV?

I'm reading more on the topic, includig Qs on SO and everyone seems to have used some bit of hackery so as to get a certain video-type encoded into FLV. Why is it such a big deal? Seems very simple to convert one format into another, but 开发者_如何学编程that doesnt appear to be the case...

ffmpeg works for me.