The fish and hooks are bigger so it is harder to get good hook penetration.
The casts are typically longer which compounds the issue above
Another reason is if you lift the rod to strike and miss hook up you have pulled the fly away from the fish and you may not get another shot. If you strip set and miss hookup the fly is still in the water near the fish and another strip may get the fish.
once you strip and fell solid weight you can lift the rod