Because the starters still try, they don't just half ass it, or at least the vast majority of them don't, and they're more talented either.
Mannion had a good game, nobody is really going to debate you hard on that fact. You're getting resistance not because people don't like Mannion or think he's bad, but because you keep trying to elevate what he does and what he's capable of, while downplaying everything that Goff does.
If Mannion was as good as you say he is then he would have been given a chance right now. The coaches and scouts in the NFL spend far more time evaluating every player on every roster, you think that coaches don't get tape of Mannion and that's why he hasn't gotten a chance? Because they do and they evaluate him.
I like Mannion, but he's not a starting QB, he doesn't have the upside of Goff and he doesn't have the skill of Goff. Every single year every team talks about how great their backup is and believe that their backup could step in and start.. The reality is most aren't and can't, which is why you constantly see a struggle to find that long term answer. Mannion not being given that shot you're asking for means that the coaches in the NFL don't seem to think he has that potential. That's fine, there's still a place for him in the NFL, but those coaches know more than any of us here.