Part of it is laziness, but I think most of the problem is a complete unwillingness to expand as a studio. Gamefreak is the only developer of a 10m+ per game selling franchise to have less than 200 developers as far as I know. Even worse, not all of their 143 devs work on a single game, they are most likely currently working on 4-5 different projects; Sword/Shield, Giga Wrecker Alt, Town, Sword and Shield's 3rd version, and possibly Gen 4 remakes or Let's Go Gen 2 remakes are already in early development. There's no denying that Gamefreak is woefully understaffed to be handling a franchise as popular as Pokemon, where even the remakes sell over 10m and the main games push pretty close to 20m.
Comparison of Gamefreak's size to other studios that develop games that sell over 10m:
-Pokemon- Gamefreak- 143 devs split across multiple teams
-The Last of Us, Uncharted 4- Naughty Dog- 497 devs (most likely currently split into 2 team, the larger team on The Last of Us 2 and the smaller on a different game)
-Horizon Zero Dawn- Guerrilla Games- 250 devs (plans to expand to 400 after moving to a new building so that they can develop 2 games at once)
-Red Dead Redemption 2- Rockstar- Exact developer count unknown, but Rockstar claims that it was a collaborative effort with work on the game being done by teams at all of their studios, and they employ 2000 people across all of their studios, so probably at least 1000 people worked on the game in some capacity.
-Elder Scrolls and Fallout- Bethesda- Bethesda staffed up to around 200 for their last main game, Falllout 4, and are currently staffed up to 400+ devs across 4 studios
-The Witcher and Cyberpunk- CD Projekt Red- Witcher 3 started development with 150 full-time devs, increasing to 250 before the game was finished, and 1500 total people worked on the game in some capacity, between contract developers and localization teams. They have staffed up for their next game, Cyberpunk 2077, with over 400 full-time developers working on the game.
-Call of Duty- Treyarch, Infinity Ward, and Sledgehammer- Treyarch and Infinity Ward each have over 500 devs now, while Sledgehammer is down to 225 after recent disarray at the studio.
-God of War 4- Sony Santa Monica- 200+ devs
-Borderlands 2- Gearbox- Gearbox currently employs 368 devs as Borderlands 3's release approaches.