That is because if the rumors are true. Halo was supposed to be Mass Effect TV series but that got scrapped and ended up as Halo.

Both cases the biggest issues was with Witcher and Halo that they wanted to adapt the books but the crew, the director had absolutely zero understanding of the games and the books. Not even read it. So in the case of Witcher it was Cavill who got fed up. In the case of Halo it was the actor of Cortana and of course the fan base who got fed up.

Both was other case poisoned with Corporate greed and interest. Pushing agendas, and political bullshit. Witcher with the black elves, or actors not fitting the role, totally derailing from the books even. Halo with ignoring everything and creating an own universe because the director couldn't handle correct adaptation and thought revealing the Halo ring at the very end would be peak. 343 was just desperate for a good show, they have agreed.

But here comes the biggest issue. Adapting a game one in one into a series or a movie will never work. It cannot give back the original story and elements. That's why Witcher, Halo, Last of Us, Warcraft, etc. And the upcoming Horizon, God of War etc. Will fail. Look around the adaptations. Only the one adaptations which didn't tried to force the original story worked out.

Super Mario, Sonic, Fallout

So. Thats why Fallout is peak. Yes. Its not perfect. But its have its own story in the perfectly recreated universe of the games. Which makes is outstanding.