In theory, what you say is correct. In practice, when you're face-to-face with these people, chatting and munching on crackers at the holiday party, and you know that you are working hard to make that guy right in front of you rich, it starts to be disconcerting.
Sometimes, even if we are OK doing job X for $Y, we can stop being OK with it if new facts emerge. There is an interesting study with monkeys (http://news.bbc.co.uk/2/hi/science/nature/3116678.stm) where monkeys who would otherwise be OK with completing a task if the reward was a cucumber, got angry and refused to complete the task when they saw other monkeys getting a grape (a better treat for monkeys) for the same task.
Yes, rationally, if you are happy doing job X for $Y, what others get out of your work should not matter, but sometimes our primitive brain objects.
It has sort of dawned on me that working as a non-founding employee in a startup is like working hard to enrich other people (the founders, some investors, etc).In a big company, things are a bit more "spread out". It's very unlikely that your work may catapult your big company into stratospheric valuation levels. At most, you will help improve the product line and increase profits, which are...