The follicles migrate.
You have the same amount of hair, but each hair just moves from your head and finds another home. A lot of hairs just jump from your scalp with no real plan in mind and drop where gravity takes them --- taking root on your neck, back, top of ears, shoulders, toes, etc. But some hairs make a more targeted journey and settle in places where they can be nice and cozy --- inside of nose, inside of ear, and other bodily crannies where they can be protected from the elements and hard to get at with a blade.
It may be that these great migrations are like the salmon, and when the hairs arrive at the destination, they find a mate and spawn, actually producing more offspring. It is possible to actually have MORE hair as you age, just not on top, which is infertile spawning ground due to solar exposure. It's quite common to end your days looking like a bald-on-top Sasquatch.