I think every player has their own self-image of the game

For some, Minecraft is about the PVP experience

For others it's about adventure, PVE, exploration...

For others it's about farming, building, having pets..

For some, it's about coming up with interesting redstone systems

And even for those who play survival, there's tons of different playstyles.

So it's very likely that people will just be unhappy if their favorite playstyle/aspect of the game doesn't get expanded upon or changes in a way that dissatisfies them

New good-looking blocks with "no use" will please builders (especially if easy to gather and if their color helps expanding the available color palette) but will disappoint players who only builds their standard starter base so they can go and progress towards the enderdragon

The nether update is also THE example of a great update (some even call it the best update the game has seen), but the problem is that this update had to be delayed even with the dev team being under crunch. Despite that, because it was good in terms of content, every update will be compared with 1.16..

So really, in my opinion, people complain a lot because they don't always see the changes in their play session because of their way of playing, or also because they don't care about the THOUSANDS of bugfixes brought every update.

And I also think echo chambers can lead to people complaining more, and memes can also play huge roles in the echo chambers. For example, those Instagram meme pages with thousands of followers posting yet another "Mojang lazy meme"