Because most people want to “do it all myself”. They choose to go vertical instead of horizontal. There are not many ways for players to improve their profits horizontally, except more alts.
Players that want to increase profit margins don’t have a lot of choices. The margins building more complex things are not that much better than building simpler things. The best way to increase the margin is via vertical models.
This impulse is made stronger with the high NPC Taxes and Fees. Why pay the VAT at each stage of production where value is added if you don’t have to? Makes perfect sense to avoid it. If CCP were to lower NPC taxes and Broker fees (although I do have to applaud them on the market ticks and ‘change order cost’) then perhaps the behavior would be lessened. There will still be the ‘middle man markup’ which would impact margins at the later stages of production, but it would be far less significant from a cost perspective.
All in all, more specialized areas are good for an economy. CCP has no choice but to impose these specializations artificially as there is no opportunity for players to discover, invent and produce efficiencies. Efficiencies that present them selves daily in the real world and have lead to the greatest fortunes mankind has ever seen.