Why do aircraft leave the cruising altitude long before landing just to circle?
It's better to be low(-ish) and ready for a spot to open, than high and far from that spot. As to why, for busy international airports the answer is really simple:

► There isn't a way to manage it near perfectly (yet).

To understand that statement, requires some prerequisites, so I'll try to simplify and summarize the basics:


There is the concertina effect: it's when fast vehicles slow down, the spacing between them go down. So the nearer the airport for landing, the more squeezing happens.
Add to that that the arriving planes come from all or most directions, and the same low-ish airspace is being used by departures as well.
The trajectory of each plane depends on each plane's load and performance, and the wind it experiences. While wind is forecast, the forecasts aren't perfect.


A solution to that is for all planes to send their estimated trajectories to the air traffic management (ATM). That is one of the projects being worked on in Europe, but it's at least a decade from full deployment. One part of it is the extended arrival management (E-AMAN).

Why wasn't it worked on decades ago has to do with the history of data communication in aviation. And it boils down to money and return of investment. Initially some of the users, like the airlines, didn't see a benefit of the high cost of the high tech solutions they'd have to add to their planes.


Seasonal adverse weather en route or in the terminal area complicates matters even more.
Busy international airports with multiple runways land planes every 30 seconds or so, so you can imagine the impact of the slightest delay one plane headed to that airport experiences. So arrival management is first come, first served.




Related: What are the negative associations with Continuous Descent Approach?