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Stronglifts was basically based on Starting Strength. Rippetoe has explained that the reason for that is he designed Starting Strength for his usual market. Which are high school kids starting out with weights and are primarily doing so because they need to get better at a sport. They also need to be able to still play that sport the day after their workout, so it was just 1 set of 5 reps.
Imo, no need to limit yourself to 1 set if you're not competing at a sport, as in you get to rest on your rest day.
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