In the 17-odd years of my deck acquisition, I have divested myself of one (1) deck; I'm the wrong person to offer insight into why people get rid of decks short of financial devastation.
I am all for selling and clearing clutter I just sold all my baby stuff on gumtree but tarot decks gosh I couldn't do it, well not right now but you never know.
As a stuff-oriented person down to my bones (who has, many times, had Le Fanu's experience of striking up a beneficial acquaintance with an item that has theretofore spent many dormant years under my roof), yes, even I feel an organic need to "downsize."
There is no doubt about that--I would be happier with less things around me.
However, there are about two thousand and one different "things" of which I'd sooner divest myself---documents that want shredding, books, clocks, old winter coats, boots, watches, an electronic keyboard I haven't turned on in 7 years, furniture, a scanner/printer I recently replaced, and half a dozen kitchen appliances I can't be bothered to use---before getting rid of even one of my lesser-liked tarot decks.
Of all the things I own, tarot decks take up the least amount of space, even en masse, and provide the greatest potential benefit in the long run.
They don't go out of season or style (not that I really attend to such things), you can get a lot out of artwork even when you don't like it, tastes in art often change over time, and anyway---decks have a built-in function independent of my feelings about their artwork. (And failing that, there are various ways to press into service a deck one doesn't use for reading or playing games).
Nope, tarot decks will be the last thing to go unless I find myself in emergency need of money for a greater good (i.e. besides my own frequent need for rent/food)--bailing out a relative from jail, for instance, or raising funds for an overseas plane ticket to visit an ailing loved one.
I can only speculate that for those who get rid of decks, "downsizing" is much more essential to their immediate mental health and well-being than it has been to mine. Or they could come from families or cultures in which spartan environs are the norm and in which having large amounts of personal possessions has never really been an acceptable option.