I doubt this is the only reason, but one contributing factor is probably that the cited articles declaring a game as "best of all time" are often retrospectives

I doubt this is the only reason, but one contributing factor is probably that the cited articles declaring a game as "best of all time" are often retrospectives from a number of years after the game released. E.G: Portal was released in 2007, yet all-but-one of its references are from the 2010s/2020s. Given another 10 years, there'll probably be at least a few more games in the 2012-2021 sections.