There's a hierarchy to everything so it doesn't diminish one thing simply by existing unless you start to compare them 1:1. Thor is a tier above say Captain America but people don't lose respect for him as a result, it's the same with this situation. Kang is a threat to reality because of his tech and his mind, he's proven that time and time again.
The Celestials, however, are on a plane of existence that's outside his purview. They don't even exist in the same way he does but just saying that doesn't give you a good way to grasp it. Example: Celestials have dogged out prime Odin and several pantheons of gods with no effort. Now keep in mind Odin alone can snuff out galaxies. Of course as with anything in comics, they adjust their powers with different arcs under different writers as the decades pass but unequivocally across the board on average they're several tiers over Kang.
To be fair, they'd do the same to Thanos and every other villain and hero introduced in the MCU so you can see why I said that they need to keep them away from anything else not Eternals related because it breaks everything.