I don't know why, but people in the lifting world has this aversion or idea that you don't have to train much to become really good at it. Its like with lifting you can get away with training like shit and not putting any real time in.
"Don't lift for more than 45 minutes at a time or you'll go catabolic" etc...
I can't speak for Bodybuilding, but for powerlifting the really great guys train a ton more than your average trainee. The competetive guys in my club (world record bench presser, 2 national winners 225 class, 2 junior winners in national bench press) train 4-6 times a week at 2-4 hours a time.
Sure, you don't need that much from the beginning, it might even be counter productive. But if you have big goals you have to train a lot eventually and take it seriously just like any other sports athlete.
SL5x5 will get you strong to a certain point but if you would train more, you would prob also get stronger, you just have to accept that the marginal benefit curve of that extra workout time starts to flatten out after a certain threshold.