Many of them are fully depreciated and worth nothing, or nearly so, on paper anyway. Any new employees won't want an old laptop. And it will cost time and money to deal with shipping, storage, cleaning, re-imaging, etc. On average, the bean counters must figure it's cheaper to let people keep them.

Worth maybe $1000, not even half a month's rent in almost any major US city

Eh, from the company's perspective this is logistically easiest--the laptop's value is hardly worth the effort.

companies that collect their laptops from laid off => where do these laptops go? how they recycle them?