There is many reasons for the album sales boom
1- Kpop is bigger now, in 2020 when BP sold +1M copies only 2 groups had more than 1M sales : BTS and NCT. Now you have more Boys Groups ( Seventeen, TXT and Stray Kids joined them + more NCT subunits). As for Girls Groups, Twice latest album is around 900k, Red Velvet (without WW distribution) 600k, Ive (single album, no WW distribution) 600k. New groups (from big agencies) 1st albums are selling 200k minimum
2- Worldwide distribution. Kpop albums are available in many stores around the worlds, more kpop acts get WW distribution deal so higher stocks orders.
3- Companies are now doing Digipack. It allows fans to bulkbuy more albums and inflate sales. Digipack have individual versions (with photocards) and are cheaper than buying a standard/photobook edition, solo stans/fanbases will likely buy this instead of a group versions. NCT have individual digipacks, Treasure too and now Aespa. More kpop groups will have digipacks in the future. There is also jewel cases for some kpop groups, like Ive, it's also cheaper.