Sure, some of the costs are (relatively) fixed. But in a mature market where there is almost no differentiation between years and manufacturers, many of them also go away due to no longer having any reasons to release, distribute and support multiple new models each year so the R&D and other costs can be spread over 2-3 years instead of being a recurring yearly expense across multiple slightly different versions of practically the same product.
And how many times would you buy LCD from a manufacturer whose models consistently fail after 2-3 years before looking elsewhere? Both of my LG LCDs started malfunctioning after about two years and in both cases, it turned out to be due to under-rated caps in their PSU. Replaced those caps and now they are into their 7th year... 5 years of extra life for a ~$0.10 difference on the original BOM to get caps rated for 2A RMS ripple instead of 0.7A or ~$1.50 worth of replacement parts. Cutting a $0.10 corner on a $300 device that makes such a drastic difference in usable life is quite insulting.
Cellphones and tablets are not quite there yet but they will be about on par with today's PCs in about three years and today's PCs are already grossly over-powered for most people's everyday needs, which is why desktop PC sales have started dropping 10-15% per year as more people go from 2-3 years upgrade cycles to 4-7 years. What effect does this have on manufacturers? Some are bailing out of the market, some go bankrupt because the market can no longer support as many players while others scrap redundant product lines, stretch product cycles, expand into different product categories, etc. to reduce their duplicated cost overheads. Cyclic market crunches have forced most HDD, DRAM, CPU, GPU and LCD manufacturers either into bankruptcies, sell-outs or mergers; future crunches will likely do the same for other components in due time.
The only reason phone and tablet manufacturers can afford wasting so many resources on product diversity is simply because the market's desire for new shiny toys with better specs has not been saturated yet. Give it another 3-5 years.