I think the reason beginners are told to do higher reps is because their nervous system isn’t conditioned to lifting heavier loads. Let me explain myself:
If a beginner does a 3x10 workout of squats, they may be able to use 200 lbs. If they try to do a 5x5 workout, they may only be able to use 215 lbs., which isn’t a whole lot more. If you do the math, you’ll find the total work output to be:
Granted, these are all just hypothetical numbers, but the take-home message is that a beginner will reach a higher work output using higher reps. As your training progresses, your nervous system will adapt to handling higher training loads, and will be better able to access the higher-threshold muscle fibers.
CT himself may have a better explanation for you, but I should mention that you’ve already done yourself a huge favor by realizing and admitting you’re a beginner, and you’re willing to start at the bottom and work your way up. Congratulations, and good luck.