I think many people overestimate the amount of Bitcoins that are actually lost forever. Many people don't want to sell them and there is also no point in sending them around. Also there is a big security concern to take them out of cold storage. So to me it is normal that many coins are not touched for 5+ years.
There are also some users in the past who already forgot they have Bitcoin, some already lose access or no way to retrieve their BTC. I knew someone whose private keys were save in his phone but that phone is already broken. Many might also experience this since they don’t expect BTC to reach it’s todays value they don’t secure it that much when they have it before. I cant even remember some of websites that I have account before when I do simple BTC mining back then and I regret that, sure many regrets also not keeping it or losing their access.