No high-income country has shown serious interest in joining the EU in decades. Norway is a hyper-privileged nation that already has several relevant treaties with the EU and has very little to gain by submitting to a system where the Balkan gets to vote on the measures Norway must implement.
Is the top not an article about a high-income country expressing serious interest in joining the EU?
... expressing some interest to try and see if joining the EU might be feasible or not. That's literally what the referendum is about.
Nobody wants to risk when things are going well, obviously. These countries do, however, have a huge number of bilateral agreements, so they find a lot of benefits in joining, at least partially, the EU. Norway already implement most of EU regulation without having any voting powers.
are there any more high-income countries in the European geographic area that haven't already joined?
Switzerland is the only relevant one really. Norway is practically a member in all but name (Not all members use euros) and the rest are microstates.
Norway is a weird one... They have things like Schengen. But some how they are not part of customs union. While Türkiye is member of customs union but not Schengen...