Simple answer: most employed people have better things to do, and don’t waste their time on participating in LeetCode weekly contest; also, this may come as a surprise to you, but for some people working at good product based company isn’t their ultimate life goal.
Longer, a bit more accurate answer: a lot of people who perform well in LeetCode contests are competitive programmers. Even without having any additional knowledge and insight, you can basically take standings, copy-paste usernames from the top of the standings into Google search, and discover that most of those are active (and relatively good) at platforms like Codeforces.
Competitive programming is most popular among students. Once people get full time job, they often stop actively participating in competitions: becoming any good at competitive programming requires investing a lot of time into practice; students usually don’t have nearly as many time commitments as adults - typical university program takes much less than 40h/week employment, having family will add more commitments on top of that. So a lot of people you see at sites like Codeforces (and at the top of standings for LeetCode events) are students, and that’s the reason why they aren’t currently employed by top companies. When I was doing competitive programming several years ago, even getting internships was often frowned upon, because it meant that instead of working on your skills and improving overall level of your ICPC team you are going to waste several months on doing actual programming (that doesn’t help you with such competitions in any way).
It is no surprise that most people visiting a site aimed at interview preparation are doing this because they are interested in interview preparation, and have some kind of good product based company job in mind. However, for competitive programmers it is often not the case. From competitive programming perspective, they basically see it as a bit weird platform with a bit weird and unusual set of relatively trivial problems - see also Bohdan Pryshchenko's answer to How can the tops finish LeetCode weekly challenge that fast, usually under 20 minutes? Some of them will be interested in working at companies like Facebook or Google - but not all. I know contestants who moved to other areas, like scientific research or starting their own companies.