Current bitcoin supply 21 million Bitcoins right now not reach yet, many people loss their bitcoin when price still cheapest with loss from website have been scam, loss access with hardware wallet until most of them forget which one place for storing their bitcoin.
Bitcoins lost in scams are not "lost" coins, they have a new owner, aka the scammer who can move them freely.
Also, all those numbers are just approximations, we have every month a clickbait-ish title about "satoshi era coins" moving again, so maybe a lot of those 10-year-old coins are not truly lost. Besides all methodologies have problems with one issue or the other, first age doesn't mean lost, and just moved 2 weeks ago doesn't mean the user hasn't lost their hard wallet in the meantime. Also, custodial services are not moving their coins often as long as they have in one wallet enough liquidity, so all that million coins coinbase is holding alongside what Bitgo has might look like 5-10 years old unmoved coins but are still pretty much accessible. And a ton more!
This is pretty much like trying to find all paintings from an artist, it's been 130 years since Van Gogh died and we're still finding one new paiting each year.