It's not "my explanation" it's the way of enterprise asset management and the general lifecycle of IT gear, leases, warranties, etc... these large companies rarely if ever sell their gear themselves too, there's an entire industry around this equipment.
I'm still not sure why this argument exists, as @T_Minus said it's lifecycle of tech equipment, every 3-5 years upgrading lead them came out cheap on eBay or many other sites, CPU, SSD, NIC or what have you, the rule applies all the time.So your explanation is "Never Attribute to Malice That Which Is Adequately Explained by Stupidity".
So you say that in many large companies the SSDs are bought or leased, then they are used during a definite time, e.g. 3 years, then they are scrapped, regardless whether they are worn out or not.
Then the availability of many little used SSDs is caused by the fact that someone responsible with their purchases was stupid enough to overprovision by buying more SSDs than actually needed.
Of course, you may be right, even in the majority of such cases.
However, it is not absurd to believe that not all such cases are due to incompetence, especially if, as you say, there is an entire industry around this equipment, so when someone buys too much gear, which will lose money for his company, then that uninspired decision will bring profits for others.