It was a thing when my 13 year old was a baby. All the brand's do cycles. Back then the popular brands were Totsbots fluffles (microfibre was this new amazing wonder fabric on the market) and RAINBOW Bamboozles (the first coloured fitted nappies). Motherease was the plain but most common nappy brand. The sell out and high resale value nappies were handmade ones from popular work at home mum businesses (WAHMS). These were called customs and you could get anything embroidered on. You'd have to go on a long waiting list to get a 'custom slot'. Once someone has waited and then got their nappy they would invariably sell it on. favourite WAHMS would go in cycles. Then Blueberries and Itti Bitti's took the market by storm and started the sell out brand thing.
When I had my next child the most popular brand was TotsBots and people would post in seaech of charts with crosses over the ones they had and circles over their must haves (want want need). The limited edition Royal Flush print (released for Prince George) and a Christmas print that had limited numbers both sold for upwards of £100 on eBay. If people aquired them they wouldn't dare use them! Heaven forbid crease the velcro tabs as that reduced the resale value. Print placement was a pretty big deal too.
After TotsBots the next flavour of the month was Bumgenius. All their prints were names after 'genius' people like Albert Einstein, Ada Lovelace, Harper Lee, a (Lewis) Carol or a Jules (Verne) were particularly sort after.
I do confess I got swept away a little, but I didn't have to have them all. The hype really fascinated me though, and I definitely noted it is the same with Slings/Wraps, prams, clothing brands (Frugi, scandi etc) all producing limited editions and there being a need to collect. I genuinely think a need to collect is a human instinct most of us have, a sort of hunter gatherer thing that is possibly exacerbated by hormones and parenting an infant. Join any buggy addict/sling addict/scandi addict/wooden toy addict and it is apparent.
It isn't just an affliction of mothers though, a lot of children and adults have a desire to collect, my mother is a knitter and has an obscene 'yarn stash' that seems common with all her knitting friends, my grandad had football programmes, for some people it's bikes or books or records or DVDs or Disney collectibles or shoes or handbags. It is often blamed on capitalism and greed but I think it is more instinctive.
People who don't fall for 'things' may fulfill their hunter gatherer instinct with savings and spreadsheets or harvesting and prepping food maybe?
Maybe I'm delusional and it's just rampant capitalism.
As for the eco thing, there is certainly truth to your point, about it not being environmentaly ideal, but a cloth nappy collector would never throw these nappies in the bin, they will be sold and resold and sold and resold. Yes this has a carbon footprint, but there are far worse things.