Originally Posted by
timcupery
(I've also seen this on even older bikes with five-speed freewheels, back when there were only five-speed freewheels, so this can't be the only reason).
One thing to keep in mind is that 5-speed freewheels commonly had a 14t small cog. That'd be the 3rd or 4th cog on a modern cassette! Because of the way the chainstays angle the designers back then might have felt the need to leave a little more space.