There's a special, rotten place in the heart of everyone who's ever tried to learn anything for the movie training montage. If only it were so easy to learn karate/drums/squirrel juggling in no more time than the length of an inspirational pop song!
Except it turns out the movie training montage is based on real-life neurology. It's actually a pretty spot-on recreation of how your brain learns a new skill.
The first time you do something even slightly complicated, you're going to suck at it because, well, you've never done it before. Your brain has no "internal model" for those combinations of muscle movements, so it's kind of like yelling at someone who doesn't speak your language: They're not gonna know what you're telling them to do, and everyone is going to get frustrated. The more you yell at your brain, though, the more likely it is to recognize the words you use.
Right, so that's basically the neurological explanation of "practice." Here's where it gets weird: All those times you yelled at your brain, and it didn't know what you were saying tend to get flushed once you've completed the learning process. Think about it: Do you remember every single time you sat down to play the guitar and sucked at it? Of course not. You just remember being kind of bad at it and then a little better and then vaguely good at it ... kind of like a movie training montage.
Your brain is like a hypervigilant Marie Kondo, clearing away anything it doesn't need through the process of "memory consolidation," so once it had a strong internal model for being okay at guitar, it discarded all those times you sucked at it. Even in a movie, no one learns karate in three minutes; what you're seeing is their memory of learning karate.
Let's just assume they were listening to "Eye of the Tiger" for six months straight.
Top image: United Artists