Very simple really because it works!
People are drawn in my the very low fare.... that can be advertised.
Very low fares are available if you are willing to have NO FRILLS and book in advance.
For those who want to add on to their basic ticket price for things like checked luggage, meals etc then they can.....
Or you have the alternative where all of these options are factored into every ticket even though they may not be used making the ticket price higher.
As for airlines having people paying for their own training.... If you ran an airline and had people throwing themselves at you to reduce what is a very expensive overhead.. would you not accept?