Because there are tiers in the league,
- bottom strugglers (yearly relegation candidates where the only objective is survival)
- Mid table dross (always safe, never do fudge all else)
- Best of rest, better than mid tabe but not top level
- Top 6
- Title challengers
Manager who are good at one of those levels don't necessarily translate to another, the pressure/expectations is a lot difference, the odd disaster result would be tolerated at one level but not at another, not being in Europe and regular early cup exits makes a hug difference in the squad management required, level of players have very different level of egos, management skills.
If you look at the top 12 spots in the league, what manager is "overachieving"? I'd argue none, the two best of expectation vs. current position right now would be West Ham and Leicester, you think Moyes is a good manager? Based on the table, I really not seeing "lots of", or in fact any managers that I would be confident would make do lots better with this squad. It' a typical, lazy narrative throwaway comment.
To your last point, yes, some managers lose it (probably true in lots of high pressure professions), but Jose is yet to truly fail, maybe it will happen here, but it hasn't happened yet.