If you really want to know:
The spread of covid is actually controlled by managing overall risk. If the collective risk of a group is low enough the spread slows down to a low level or if it's high enough the spread speeds up to a high level. Overall, higher risk activities can be balanced by lower risk activities and still control the spread, so some higher-risk things can be allowed if they are uncommon enough and valuable enough. So, e.g, suppose someone figured out that allowing plane travel and medium-size gatherings were about the same risk but we could only tolerate one or the other but not both and still control the spread of covid. Then it would come down to which was more important/valuable.
Of course, this is complicated, and involves assessing and balancing the relative risk and importance of many more things than those two, and needs to be boiled down to something that can be effectively communicated to a mass audience.
> I feel like the CDC has this mistaken idea of how to get trust, that they will become trusted by always erring on the side of "more lockdown"... like the boy crying wolf...
No, in fact, the contradiction you see in the different recommendations about medium-sized gatherings and plane travel shows that the CDC is balancing their recommendations, some more restrictive and some more permissive.
I can see where you're coming from: you want there to be simple and clear recommendations based on simple and clear criteria. But the problem -- balancing risk across a society -- is inherently nuanced and complicated. The more simple and clear you make the solution, the poorer fit the solution has to the actual problem.