The autobrake system on most aircraft does not start working untill after landing when system logic conditions are met, one being reverse selected.
The increase in landing distances is for the time delay with the autobrake system logic ref the above.
In english - immedately selecting reverse after landing is sending a logic signal that you want to stop. One reason not to delay deployment with (or without) an autobrake system.
Max autobrake logic I suppose may not require reverse selected. Or at least the time delay is very much reduced. (thats a guess)
Rgds