Maybe the audience of FOSS is "self" and therefore I will obviously write something that matches my need.

Can you provide one example ?

because gnome is FOSS and I have heard complaint it's 'too simple'

and https://birchtree.me/blog/screwing-up-the-simplicity-of-it-a... : "Most people get maybe 2% of the potential of their Macs and Windows PCs today. Have you watched most people use a computer laterly? Most people I see have all apps in full screen all the time, no matter how big their screen is. Most people I see use keyboard shortcuts for copy and paste, opening new tabs, but basically nothing else. I've seen numer ous Mac users download apps, open the DMG, and run the app from the DMG forever because they don't know you should move it to the Applications folder. Many people have a desktop full of files because the desktop is the file system."

(and SAP and any Oracle Enterprise products...)

see this sub-discussion on UX of b2b products: https://news.ycombinator.com/item?id=29731828

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?

Garbage in, garbage out.

I think most software engineers are (in a way) glad that they don't have to talk to customers, and that there is someone in the middle. Managers and such are tolerated by developers for this reason.

The biggest problem with this stance is that far too often those middle-managers or other customer interfaces understand neither the customer's actual needs nor the programmer's needs.

Every successful project I've been part of has required me to meet directly with the customer and understand what isn't working. Asking someone else to ask the customer for a specification just leads to the wrong solutions being solved or an endless stream of feature requests that make no sense in isolation.