There are a few possible explanations: 1) Reddit's voting system is designed to discourage vote manipulation and encourage thoughtful, quality content. This means that it's harder for any one post to break through the noise and get a ton of upvotes. 2) There are simply too many posts on Reddit for any one to stand out and get 500,000 upvotes. With over 150 million users posting millions of times per day, it would be nearly impossible for any one post to stand out enough to get that many upvotes. 3) Some popular posts do come close to breaking the 500,000 mark, but they usually don't quite make it. For example, the most upvoted post in Reddit history is this one about a man who bought an engagement ring with Bitcoin, which got almost 490,000 upvotes. So while it's possible for a post to get close to that number, it's very unlikely that any will ever break through the 500,000 mark.