This is the employment equivalent of Betz' law from wind power. Before you throw that idea out as ridiculous let me explain.
Betz' law states that there is an upper limit to the amount of power that you can extract from a given mass of moving air (aka wind). The plainest explanation is that the wind has to go somewhere in order to be able to bring in fresh wind. If you extract all the energy then the air would have to pool around the windmill and then eventually the pressure differential would drop to 0 and there would be no more wind. So you give up some of the energy and use it to remove the air after you're done extracting energy from it.
The parallel is that if up-or-out wasn't a fact that these companies would sooner or later run out of room in their org chart to bring in fresh recruits at the bottom. This would stop them from hiring the occasional gem that they will promote to the top of that chart. So to be able to hire the really good people they have to create room at the bottom, they have to get rid of a certain percentage of their people at all levels every year to keep the machinery moving.
It's an expensive affair but in the longer term it makes good business sense. Both windmills and really big companies are extraction devices.