What is so hard about making an AWACS?

For the past couple years now, whenever the discussion of air capabilities come up, be it in a real conflict (Russia-Ukraine, India-Pakistan) or geopolitical shift (US leaving NATO), the topic of AWACS always comes up and how it's one of the major reason one side or another underperformed in such a conflict.

Yet, from my uneducated point of view, it doesn't seem particularly that difficult to make one, especially with AESA being fairly ubiquitous in the military world now. Half of the ones in service just seem to be civilian aircraft that an array bolted on the top like ERA on a tank in Ukraine.

What am I missing?