It's a lot like SEO optimization and the chicanery that goes on there: Airlines know that customers (a) are using ticket comparison sites like Kayak and Google Flights, (b) customers sort by price, from lowest to highest, and (c) as with Internet search results, customers tend to click on links on the first page w/o scrolling.

"Unbundling" options permits non-ultra-budget airlines to move toward the top of that list, and then make the money back by offering extras during the checkout.

Now, if sites like Google Flights and Kayak would give customers the ability to tick checkboxes saying, "I want a ticket with these unbundled extras," and sort according to the final price, that could potentially re-arrange the field, but who knows? The airlines would probably find another dodge around it.