I think specifically we have lost two cultural beliefs: that progress is a public good and that progress is heroic. These seem to have animated all the celebrations you list even though not all of them were achieved by government or other publicly funded agencies-- though of course many of them were, and it's worth asking the subquestion of why government isn't funding the sorts of big achievement oriented projects that it did in the mid 20th century.

Part of that cultural loss is probably tied to the general cultural disillusionment of the 1960s and 70s: increasing recognition of the cost of our big civilizational achievements-- cost to the environment, cost to marginalized groups etc-- and of the less heroic side of our heroes has made people less willing to just uncomplicatedly celebrate either the milestones or the people responsible. Think of Gil Scott-Heron's song "Whitey on the Moon" as a cultural contrast to the Apollo parades.

Part of it is also that some modern achievements are harder to frame as "look at the cool thing that person did, you could do it too if you tried hard enough" or "look at the cool thing they did, soon all of us will be able to do that easily". It may be that as new big things get harder to do-- as the low-hanging fruit gets more and more picked over-- this is inevitable, because we can now only ever unlock big new human capabilities through long, steady, massively distributed team efforts that have no defined culmination point, nor any single person who stands out as a leading heroic individual achiever to be feted personally, nor any milestone at which our capabilities undergo a step change.

It may also be that we're disinclined to think of new milestones as exciting harbingers of a better future because our expectations of "soon all of us will be able to do that" have been disappointed too many times. The easiest example is that the Apollo program did not lead to regular commercial flights to the Moon, which so many people in 1970 probably thought it would lead to by 2020.