system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Can someone explain what it is about music theory that allows pretty much every song to be mashed with eachother in a way that fits?

I know nothing about music theory or how songs are constructed, beats, bars whatever. But for the last few years mashing every song with all star or we are number one has shown that songs with completely different genres and energies can be mixed together pretty well with just a few tweaks, what is it about the process making songs that allows that to be true?
gpt
Songwriters are unimaginative. They use the same stupid chord progressions and scales and rhythms over and over and over and over again. 

The energy of a song is largely independent of the scale/harmony, so that's no barrier. Key doesn't matter because you just shift one song or the other up or down until the two songs match. And because we all use the same stupid scales, the chords often barely matter as well. The melody from All Star is mostly pentatonic which means it works with *virtually any* typical Western chord progression. 

There's a reason this is done with mostly pop songs and not, say, King of Limbs-era Radiohead.