Great topic, I'm multitasking so my answer is rambling buuuut -
I think part of the reason ENFPs may feel "so fake" is that being authentic is very important to us. Being "real"/being "authentic" it's highly valued not just in ourselves but in general in life. It's also what makes ENFPs generally very open minded and accepting of other people, curious about the world, and random- we can pretty much accept that people are people as long as you show us who you are.
Everyone wants to feel "at home" but that home is a little harder to fit for an ENFP who is, for lack of a better word, so random. That hyper-Ne makes us wonder "do other people feel like this? Is this 'normal'? etc" And that imagination makes us wonder what could be - what if I were like this? Or this? Or this?
In some ways ENFPs are forever children inside, which isn't entirely bad, youthfulness is considered a positive trait.
ENFPs are considered "random" and getting into all kinds of stuff. It's the Ne/Fi combination (and being Ne dom and not Fi dom makes a world of difference between ENFP and INFP)
ENTPs and ENFPs are very similar in the way we extrovert and appear to the world. However, I think ENTPs are more into the experience and satisfying curiousity whereas ENFPs are more into the underlying "meaning" of everything.
And I do agree that it's a maturity thing - meaning the more you know who you are and what your values are, the more authentic you feel and the more *grounded* you feel as a person.
Not all ENFPs are born nor develop the same, I think also natural curiousity and open-ness has a lot to do with it. Because ENFPs often find themselves in different contexts hanging out with diverse crowds, the combo of that Ne checking the environment and checking it back with Fi or just self-consciousness - yeah I can see why people might feel fake.
I actually don't feel fake and haven't felt fake in a while. Sometimes I feel like I'm not doing what I'm supposed to be doing or being who I am supposed to be, but that is more frustration than fakeness. Basically, it's not that I'm acting like something I'm not, I'm just not fully being what I want to be.
I don't know if I've felt "fake" - rather I've felt out of place and wondered why. So I will say that I've felt very much like I was 'searching'. It's an inner journey more than an external one.
Fakeness really does disappear with confidence and confidence comes with a lot of work, questioning, and experiencing.
Is that generic enough?
LOL.
BTW, a lot of people think that extraverted feelers (Fe doms and EFs) are "fake". Haters! LOL. However, in general people, especially extreme introverts (generally crabby or antisocial or just very quiet) have told me the opposite - that they find me very sincere and genuine and they even enjoy my chatter because "it's not the same kind of talking that people do when they talk too much" lol.