So much of this impression is shaped by the narrative that the techblogs are choosing to cover on any particular week. In reality, all these companies have some level of attrition, and people leave one job for another for new challenges. Dropbox has 315 employees[1]. That constraint alone means that the number of slots to fill will be very small.

(I also personally wouldn't really label Dropbox as "bigco" label like Google and Facebook, if only because you can still fit everyone in one medium-large room. I guess it's not a startup anymore, but I don't think anyone would consider it a lumbering corporate behemoth).

[1] https://www.dropbox.com/about