This is a pretty standard document length. Modern microcontrollers have similar lengths (e.g. ATSAMD51 is ~2000 pages). Some of it is not software related, things like pin outs and electrical and mechanical descriptions.

It does take a huge amount of work to write and maintain. Typically the authors are not technical, so it also relies on the designers being available to answer questions as well. Then there’s a choice of how it’s written: narrative and potentially imprecise but readable, or terse and precise but hard to read. There’s both styles in the same document, terse for register descriptions.