Language changes slowly but constantly: words evolve new sentences, new phrases are coined, others become less current. The OED is so big that the only practical way to revise it is continuously, a few entries at a time. Every quarter there are hundreds of changes:
https://public.oed.com/updates/ . As with software, it turns out that financing a product that needs continuous updates is more effectively done with a subscription than by selling products as one-off transactions. (FWIW the print version of the 2nd edition OED is 20 volumes and is listed on the OED site at 860 quid.)