I find I am reduced to going to FlightRadar24, essentially an enthusiast's website, to find which carriers operate where. But if you are after a flight which doesn't go every day then it becomes just impossible.
It's not as if the airline IT department could not produce a good lookalike pdf timetable automatically from their data. Some did that, but bizarrely threw them away. Even those got ruined by including all sorts of ludicrous connections and equally ludicrous 10 lines of codeshares on the same flight. Yes, I know some flights may change. But most don't. I'll take my chance that when I come to book it, it may differ. As carriers seem quite happy to change thngs after booking but before departure, they can't be too fazed by that.
It's like going to a restaurant where, instead of being able to see a menu, you are asked "How hungry are you, when will you next eat, how much money is in your pocket ... etc", and then THEY tell YOU what you are getting and how much it will cost (we'll ignore for now "Do you want to sit at the same table as the rest of your family", a separate topic).
Basically it came because the schedule detail is just a hack of the booking system, and it has been ignored that anyone will want to see the flight availability before they are ready to book.