There are also no deeply invested customers for most FOSS development. It's not really surprising that most FOSS design feels aimless when the developers only need to meet (or aren't funded beyond) their own needs -- the bar is way lower. I don't think the mere existence of "managers" or "designers" is the critical factor.
Also, your original statement didn't say anything about FOSS:
> [...] most software engineers are bad at creating user-friendly products unless someone with product design skills tells them exactly what the UX should look like, [...]
The software engineers I've worked with want to make user-friendly software that meets needs. You know what usually gets in their way? They're not able to work with the end users to understand what the destination should be. They're fed streams (tides!) of loose feature requests with no apparent cohesion. What kind of software do you expect to get out of this process?
Also, your original statement didn't say anything about FOSS:
> [...] most software engineers are bad at creating user-friendly products unless someone with product design skills tells them exactly what the UX should look like, [...]
The software engineers I've worked with want to make user-friendly software that meets needs. You know what usually gets in their way? They're not able to work with the end users to understand what the destination should be. They're fed streams (tides!) of loose feature requests with no apparent cohesion. What kind of software do you expect to get out of this process?
Garbage in, garbage out.