They are very low-cost for some items that are traditionally high-cost. And for all the people who have trouble doing assembly themselves, most (nearly all) have a family member or friend more than willing to help. This means that even though the same person buying and owning may not be building, the popularity remains. It is also more than a meme at this point, is a specific bonding time between friends and family alike.

Edit: I forgot to add, I think everywhere IKEA also offers an assembly service, even same-day if you do it early enough. And with the assembly cost added, I expect their furniture is still cheaper than much of their competition locally.