I agree! It varies from one airline to the next and part of it is - that schedules are now changed far more frequently than when I started paxing in 1965! Carriers may alter a schedule and not want to draw attention to it. They may add more rotations or change times of rotations during the season. Not often, but it does happen. I suspect another reason for not giving comprehensive long date ranges in PDF/searchable form is that - by doing so they are telling their competition what they are doing.
If I want to know a specific airline's timetable, I pick a week in the general time span (allowing for high/low season) in which I am interested and then note what it says and, yes, sometimes you have to sample 7 times. I was doing this just last night and found that BA gave a week at a time for the destination, whereas Virgin Atlantic gave only the single date, eventhough they operate the route 7 nights a week. I also found that they (VS) had changed their flight numbers for the destination. This was irritating as they had stayed the same for over 20 years and I knew the numbers for quick searching.
If I want to know overall options between two points, I use one of the (too numerous) search sites and select 'direct flights only' and ignore all the pricing - just the carriers and airports. This will tell you a lot and you can then narrow it down but there is no easy, free, shortcut that I know of.