That's true for large urban areas like Oslo. However, the small tourist towns in Vestlandet, Norway, have some shuttle-sized hop-on-hop-off buses. Or at least had them when I last lived there circa 2016. And in Klaipėda, Lithuania, the mini-buses are regulated and integrated into the public transit system. Where there isn't a large urban transit demand, these mini-buses serve a meaningful function.

I think the circumstance that they pop up "when public transport sucks" is seen more in the US. Jitneys are considered "paratransit" there — fundamentally a substitute. In many Eastern European countries, a common issue was that marshrutki cannibalized existing public transport options by duplicating routes (more on that in the Wiki article I linked in my parent comment). They compete more as equals, not fill an under-served market niche.

By the way, a marshrutka serves one of the last NATO-Russia routes[0]; a very meaningful route in both public transit and diplomatic, cultural contexts. I will concede to you that this is a case of "public transport sucks" to the highest degree, on a global scale.

These route taxis are very versatile, and the diversity of how they are used and their relationships with public transport is huge.

[0] https://www.youtube.com/watch?v=8GIxov7xVxo