Why that is is because, in my view, part of the GM's role is to set the parameters and rules of the game...though of course this can also be done by mutual discussion and agreement among all; which means the players come into it as well, muddying the waters even further as to who "says".
Note I'm talking here largely about what happens before play even begins, when the GM and-or group is deciding on a system and-or if-how to modify it.
Pretty much, yes; people could put their own stamp on things and see if they could do them better. And some in fact would make improvements, while many would not.
Obviously things like copyright and patent laws etc. make much of this merely hypothetical. But take, for example, a band doing a cover of someone else's song. The new version could be better, worse, or note-for-note the same as the original, but it's not the original: that cover band has taken that song and put their own stamp on it, even if they didn't create/write the song themselves.
If I had the (probably ridiculous amount of) money it'd need, I'd love to be able to take an ordinary car and get it modified such that it more closely does what I want it to do in the manner I want it done.
Why TTRPGs are unique here is that - in comparison with art, music, cars, etc. - many (most?) people do have the chops and available means to modify them and make them more bespoke to their own wishes/needs/desires.
And further, where even in their infancy most games - and most products in general - are presented and marketed (intentionally or otherwise) as "here it is, take it or leave it", and thus that's what we as consumers have become accustomed to; RPGs - particularly D&D - were initially presented as more "here's a mostly-complete framework, make what you will of it"*. They expected and in some cases required modifications and additions in order to become a) bespoke to the table and b) in some cases, playable; and that process would almost universally be done by the GM either before play began or ad-hoc during play as issues arose. (remember, in theory the DMG was off-limits to players)
And so, the philosophy of RPG rules - in rather stark contrast to the rules of most other games - being seen as malleable to suit each table and-or GM came about; helped immensely by the fact that all it took to do so was a pencil, some paper, and some thought and time. Thus, in some ways the GM and the system became one and the same.
* - Gygax in his 1e DMG was famously conflicted on this, arguing for uniformity in some places then exhorting DMs to kitbash the system in others; looking at it now I suspect this conflict arose from an inability to square his own tendencies as a designer/kitbasher with his/TSR's desire for uniformity in order to sell more books.
Is the above what you seek? If not, please clarify.
Not sure if you're saying that "experts don't feel that they are experts" or that "experts think they're more expert than they are". In either case it doesn't much matter; the key point is the ability to be - and feel - able to do these things rather than the quality of the result as seen by anyone other than the doer.