A lot of folks think "Cliff plays simple repetitive parts" - I could do that.
OK, so let's get real. Your next task is to pick up a live AC DC album, and play through the whole thing - twice (so it's as long as a concert set). Play as many notes as Cliff, play the same repetitive lines, and play with intensity. Somewhere in the first few songs, you'll suddenly realize "this isn't easy" - there's an endurance aspect to playing a lot of notes over and over that Cliff can do for an entire tour, and you probably can't do that for half an hour without getting messy. Next time you watch your favorite YouTube bassist and thing "they're awesome", remember that he or she did one good take (out of how many?) that lasted a few minutes. Cliff plays for a couple HOURS of intense, fast, repetitive lines without messing up, and does that every night for months. He does that while jet lagged, staying in hotels, eating strange food, having not slept in his own bed for the last 6 months.
I do church gigs. One of the groups that writes a lot of the tunes we do does a lot of lines that are sixteenth notes - for long runs within a song - a couple minutes at times. The bassist is a small, skinny Asian woman, half my age, probably half my weight. A long scale bass looks positively huge on her. You'd think that she'd run out of gas pretty quick, yet she can play circles around me endurance wise. I play what I consider to be great bass lines (and the folks that call me for gigs keep doing it), but when push comes to shove, what she does...I can't do.
Simple and repetitive seems easy, until you try doing it for real.