Because the volume of search results is otherwise unmanageable - simple search queries quickly produce many millions of potential routing results, each of which would then has to be fared for display. ITA has always, at heart, been an exercise in trying how best to manage the unmanageable, while allowing the user to specify some very specific parameters to help ITA produce results that are exactly tailored to the individual's needs.

If you don't want to put the effort in to learning how to use ITA to your advantage, you can always try some dumbed-down search engine instead. Google Flights, for example.