I think a lot of it has to do with the cost; software is more or less free to create. So the barrier to entry, hardware-wise, is much higher and requires more capital.
Many of the hardware projects, and definitely most of those listed in the original post, can be built with that much worth of parts.
EDIT: But that's just the prototype. After that, you need to build the real product (scale, hardware needs factories for that) and sell it (again easier with software). But this doesn't mean we should only do software because it's easier; otherwise we would be like the proverbial drunkard who kept seeking for his money under the lamppost, because it's brighter over there.