Well, IMO most people either have to plan their vacation around some event that is fixed pretty far out (like school vacations) or else have some flexibility in timing once they know what their window is. A lot of people don't realize that booking award travel is different than booking other travel, or they do realize it but stubbornly act the same anyway.

A few people really do find themselves in a situation where they get stuck with fixed dates, but don't know what those dates will be until shortly before travel. For them, award travel really isn't a realistic possibility, and they end up using their miles for upgrades, lounge memberships, hotel rooms, etc. Hopefully, their job has compensating good features to make up for this undesirable feature.