"A lot" of heavy lifting has been done for cell phones, sure - but that doesn't mean these companies don't employee hordes of engineers, both hardware and software, to keep it moving forward. Motorola may have invented the cell phone in the 70s, but do you really think that's the same technology we're using today? Similarly, iOS is built on OS X which is built on BSD - that certainly doesn't mean Apple had an "easy" job writing iOS because the guts were already written.
Lets take HTC for an example. Lets say there are 100 hardware engineers, 50 software engineers, and 50 testers at an average of, oh, 100K total cost to the company. Those people alone would cost the company $20 million / year, not including PR, marketing, support, sales, etc. They need to make that amount on top of the hardware costs just to break even, let alone make a profit. And frankly, that employee count is assuredly low. I'm sure they have more than that.
Like I said...the better question is "why is the iPod Touch so cheap". I'm quite positive that, all things considered, Apple loses money on those...but they do it because A) they make money on music / app sales and B) it gets people roped into the ecosystem so that they might get an iPhone next.