Death Stranding came out in 2019. Death Stranding 2 will come out in 2025. That's six years for two games. If these were the PS2 days Kojima would have cranked out 3 Death Stranding games by now and we'd be getting Death Stranding 3: Subsistence along with a teaser for Death Stranding 4 next year.
Remember when video game trilogies used be to a huge thing in the sixth and seventh generations? You'd typically get 3 games in about 5-7 years
See: Halo, Gears of War, Resistance, Killzone, etc
I do think they (Kojiprod) didn't even consider a sequel until after COVID. If I remember well, in some interviews Kojima mentioned that the first game would have been completely different if the lockdown had happened during development time, and it is implied that DS2 is a direct response to it.
Now here's the thing, even with that information in mind (let's say Kojima starts writing in 2021, and preprod follows), they should still not take as long, because the core systems are already there, so I'm not sure what's taking so long. My guess would be scanning/filming for cutscenes, because at this point it's going to be more of a movie than a game
That's a good point, I imagine the new environments take some time to build too.