You're right that the "vertical slice" approach has been used very successfully in games development, though. Mark Cerny[0] has been evangelizing[1] the idea that preproduction isn't over until you have a "publishable first playable" (a.k.a., a complete vertical slice) of your game.

Basically, you shouldn't switch from "preproduction" to "production" until you can show (A) here is actual gameplay, (B) it is, in fact, fun, and (C) you know how to actually implement it. Until you can demonstrate those things, how are you supposed to estimate how long it will take to build? Or that, once you're done, whatever gameplay mechanics you dreamed up are actually entertaining to a player?

[0] arcade game programmer, producer / studio exec who got Insomniac and Naughty Dog to scale beyond their founders, & eventual lead architect of the PS4 and PS5

[1] https://www.slideshare.net/holtt/cerny-method, https://www.youtube.com/watch?v=QOAW9ioWAvE

> You're right that the "vertical slice" approach has been used very successfully in games development, though.

Oh I know, I worked in game development for years :). Was really just trying to get a short description.