Don't know the reason but to advance the science (BS) a little bit I went out this evening and compared riding clipless with platforms. I did the same route and efforts, first with a set of platforms, followed by clipless pedals. I used an SRM to record power and zeroed the meter before each set of tests. I did this with an open mind and went all out on the sprints and steep hill. (I'm 50+ and not a sprinter so no wisecracks please)

For normal steady riding I didn't find any noticeable difference. My foot obviously moves around more with platforms and I don't find it as comfortable but I wouldn't have any issue doing a long ride with the platforms. The sprints were, on average, about 200w less with the platforms and the steep hill about 150w less.

One of the most noticeable differences was just starting out from a stop. With the platforms I couldn't go much above 400w but with the clipless I routinely start out briefly around 500-600w and it was very easy to feel the difference in pulling up. Not a big deal but I suspect this is why track sprinters need their feet firmly locked to the pedals.

Results are summarized below: