It's entirely ego. This "I made it for myself uwu you don't have to like it" attitude is just a way to shrug off criticism, and even self-publishing is still
publishing, which is a shiny word that people like put on their resumes (though for the same reasons you pointed out it is losing its value). These people want all the prestige of the establishment without the skill or effort.
If you want to make something for yourself but still feel compelled to share it for some reason, just post it online. And on the Internet you can find
someone who likes whatever you're doing. Try
Wattpad,
Royal Road, fan fiction sites, make your own neocities page or wordpress blog. There are ways to generate revenue from these sources, so the money argument falls short, especially when you consider most people skipping the slush pile do so by paying vanity publishers. I think indie literary magazines also have the chance to become more prominent because of how unregulated the current market is.
There are ways for writers to put their money where their mouth is, but the draw towards traditional publishing shows their intentions aren't really to "write for themselves" but to stroke their own egos.