10–15% is about the maximum amount of dead-weight employees (the kind who don't have the skills to perform the job-role they're hired for, who were hired due to favoritism, bribes, blackmail, or pure hiring-process "throw a dart" laziness) you can have on a corporate balance sheet before the aggregate lack of expected productivity per OpEx dollar starts to be suspicious. The further below this limit a company is operating at, the 'safer' hiring managers can feel in making arbitrary/capricious decisions without being called out on them.

And so — presuming at least some hiring managers in each bigcorp are unethical and lazy or able to be manipulated — 10–15% dead-weight becomes an equilibrium point: the number of dead-weight employees asymptotically approaches 10–15%, where firing one means being able to hire one more, and hiring one means being unable to hire one more.

(Which isn't to suggest that there's top-down awareness in all these companies of these employees being dead-weight; rather the opposite — they survive because the structure of these companies has no continuous visibility into employee productivity. But when they consider layoffs, that's the time to do a point-in-time productivity audit... and that's when they find that this dead-weight has been accruing, and set out to burn it off. And, if they're smart, to also "burn off" the people who were willing to commit malfeasance by hiring/endorsing them.)