The answer can be summed up in one word: "privacy".
There are two forces at play here. One side wants privacy. When they give data to Google Calendar, they don't want Google Maps or Ads know about it. The other side (your opinion above) wants more integration between services.
In this political climate, the privacy side has an edge. This means if Google Photos want to access data on Google Calendar to provide the integration you asked above, they will have to jump through multiple quarters of privacy reviews, with a very high odd of being shutdown.
> All these big tech companies seem to just give up on any kind of significant innovation as soon as they reach a certain level of monopoly on their market
After I see how the sausages are made, I think claims like these are naive. It's worth learning more about the factors at play before criticizing something. More often than not, the agents are acting pretty rationally based on the situation.