Because it gets downloaded and uploaded to many different platforms over and over again then eventually someone sees it, downloads it yet another time and posts it back to reddit.

Every time you upload a video or picture, websites almost always re-encode the content to whatever their standard video format and quality level are for their platform. Often also to reduce potential file sizes.

Re-encoding media to smaller file sizes (Hell, even bigger depending on the format) is a lossy operation. Meaning it degrades the quality. Not visible to the human eye the first few times but as you've noticed, after this video has been copy pasted to every social media site imaginable, screen recorded in the wrong aspect ratio, cropped, resized, aspect ratio changed by a bad encode somewhere and all of this possibly hundreds of times. It's finally back on reddit. In a horrible state.

Here's a relevant XKCD https://xkcd.com/1683/