For what reason is the Czech ř hard to pronounce for most foreigners?

It’s a genuinely difficult phoneme to articulate. Back in the 80s, when the Guinness Book of Records was more than a picture book, it was listed as the most difficult to acquire—kids are supposed not to pick it up until they’re 7, and our own Zeibura S. Kathau says they have cram schools for it.

So what’s the deal with [r̝]? (See: Dental, alveolar and postalveolar trills)

• Trills are hard to articulate to begin with. As witnessed by questions here on Quora.
• There’s two articulations going on at the same time: both fricative and trill. That’s a much harder task. Much harder.
• And bugger me if I can hear anything but [rʒ] in the Wikipedia recording. Like any learner of Czech. Though I’m notorious for having a tin ear.
• It’s a complicated articulation, and (cause–and–effect) it’s very infrequent in human language, so it’s not like lots of people get exposed to it outside of Czechia. Kobon language has it as well, but it’s only one of like eight allophones of /r/; so if ever you have to learn Kobon (10000 speakers, which is huge for Papua New Guinea), you could get away with mangling it. Whereas in Czech, ržát [rʒaːt] (‘to neigh’) and řád [r̝aːt] (‘order’) are a minimal pair. Nice one, people of Czechia.