"Why only certain times of the year? Predators always eat fish don't they?"

Yes but they don't always feed the same way, hence changing up your presentations. Casting and stripping is good but doesn't always work, and not all hits are arm wrenching.

I was fishing streamers last night for stripers and just dead drifting my fly with the current keeping just enough tension on the line to feel my fly. Hits were just a slight change in tension that was all. The mighty fish that people say you need to sling and zing and strip like hell to get them to hit to rip your arm off were feeding 5 feet from shore like trout in current just lightly mouthing whatever came their way, just slowly drifting, not swimming 40 miles and hour.