The Beatles 1 is the best-selling album of the 21st century , which according to Wikipedia (estimates vary depending on methodology, but this is probably the most authoritative source), is over 31 million units sold since its original release on November 13th, 2000. This also makes 1 #17 overall of best-selling albums of all time. It’s also among the fastest-selling albums. According to Apple Records, which produced the album, 3.6 million copies were sold in its debut week, and 13 million in its first three weeks. Although, as a caveat, compiling this data is not an exact science, and figures vary depending on geography and if digital downloads are included. However, it’s indisputable that 1 was a massive success.
This never made sense to me. 1 is compilation album of the Beatles’ #1 hits–there is no new material at all, yet it sold so well. Was Beatles music so hard to come by that somehow it justified so many sales? By 2000 the Beatles had already sold some hundred-million albums. It’s not like there was a shortage. There were many best-of Beatles albums from the ’80s and ’90s, yet those didn’t sell nearly as well. From an economics perspective this seems irrational, the equivalent of finding hundred-dollar bills on the sidewalk, or in this case, millions of them. Like the ability to print money out of thin air.
Part of this is nostalgia. For the past 2 decades there has been something of a boom of geriatric acts, who attract huge audiences of new fans, and also older fans who want to relive their youth. Decades-old acts are still bringing in millions of dollars annually in touring, merchandise, and digital and physical album sales. [Genesis’ The Last Domino? Tour grossed $100 million from 2020-2022.] Or convenience. Obviously, it’s easier to buy a single album than having to compile the catalogue piecemeal. Also, it’s not unprecedented for compilation albums to do extremely well. For example, the Eagles Their Greatest Hits sold over 40 million copies, the fifth-best selling album of all time, surpassing 1, although it was released in 1976. Same for soundtracks, which are also compilations, notably The Bodyguard, which sold 45 million copies as of 1992 (#3 in total album sales).
I think another factor, relating to economics again, is scarcity. There is no shortage of aspiring musicians and acts, but there will always be one Beatles, one Led Zeppelin, etc. As world population grows and emerging markets, such as in China and Brazil undergo development and are exposed to Western culture and tastes, this canonical collection of music remains fixed.
The Beatles, intentionally or not, have always kept a tight lid on their supply. There is no such thing as a ‘Beatles reunion tour’. After acrimoniously disbanding in 1970, that was it. Each member went about their separate ways with successful solo careers, but the Beatles was no more. Nor did surviving members ever collaborate to produce new material, at least not under the title of the Beatles. Compare this to the Rolling Stones, which still tours. Most extant acts tour a lot. The Beatles even during its heyday toured little. From August 29th, 1966, to 1969, there were no live performances at all, save for the January 30th rooftop concert, in 1969, which unbeknownst to the audience that had gathered to watch this impromptu performance, would be the Beatles’ final performance ever.
Original Beatles songs are also sparingly used in commercials or movies. In 1987, Nike was sued for $15 million for using ‘Revolution’ in a TV ad. Only as recently as 2008 was the Beatles’ back catalogue available for TV commercial licensing. The tracks must be re-recorded by other artists, and not without consulting Paul McCartney and Ringo Starr and the estates of Harrison and Lennon.  Had the Beatles toured more often or had more product placement, the brand may have been at risk of being oversaturated or overexposed. There would not have been so much anticipation for 1 even though it is old material.
 This depends on the chosen cut-off date. If Jan 2001 is the cut-off, not 2000, then Adelle’s 21 is the best-selling album, at 31 million units sold.
 Michael Jackson famously bought the rights to Beatles’ music in 1985 for just $47 million, outbidding McCartney and ending their friendship. Jackson would sell half of his stake in 1995 for $100 million to Sony, and the remaining half in 2016 for $750 million at the behest of the Jackson estate.