Most of people heard about Bitcoin in 2009, 2010 or one or two years later, easily thought it is a joke, it was created for fun, it will die.
I don't agree, especially when it comes to those years when few talked about BTC considering that it was practically worthless. It wasn't until 2013 that Bitcoin somehow gained attention when it first reached the price of $1000, and it wasn't until the end of 2017 that it started to enter the mainstream.
That explains why there are potentially millions of bitcoin lost because early adopters did not care too much about their bitcoins that were easily mined with laptops or bought with very cheap prices. Who spend money to buy bitcoin with prices in cents possibly did care more about their investment and had less probability to lose their wallets, bitcoins.
People who tried and mined Bitcoin in early years have high chances to lose their wallets and bitcoins because they only tried for fun.
Stories of millions of lost BTC are just speculations because no one has ever proven it. If someone concluded that x million BTC was lost because they didn't move from an address for x years, then that doesn't make any sense. I have coins that haven't moved for almost 10 years, but that doesn't mean they're lost because I have private keys with which I can send them to anyone at any time.