Europe also has nowhere near the elite population that the US has -- I've never exactly put my finger on why -- but look at a European elite preboard on any of the *A carriers (though I have the most experience with LH) and it's essentially nil.

If you opened UA lounges up to all elite levels (or even just gold and above) the only way to make that make any kind of financial sense for UA would be to hike the qualification requirements considerably. The incremental cost of a CPU is virtually 0 and entirely in controlled food portions (since F cabins are usually over-catered slightly anyway); on the other hand the incremental cost of a lounge visit is relatively high as you have unlimited F&B consumption, greater stress on agents for travel assistance requiring higher staffing, either the additional cost of extra lounge square footage/furnishings/depreciation or the impaired goodwill from people ticked off that they can't get into the lounge because it was full (just look at how many people are ticked off based on the COVID lounge situation and that's only people who have a membership or an international entitlement--add a 30k+ golds, platinums, and 1Ks to that mix and...why wouldn't I just wait in the terminal at that point.