Folk:
Rich and long history since it dates back centuries and came about during a time when musical genres weren't as fleeting and/or splintered as they are today. It serves almost as a historical record of several long eras that have formed the cultural foundation of the various countries the respective styles of folk come from. On the other hand, it's difficult to know what "folk" even means by today's standards since I doubt what we call "folk" was referred to as such in its own times as it probably had several different genre names depending on the time period.
Punk:
Although I'm not a fan of it at all, it does contain some aspects that make it very easy for non-fans like myself to not have that negative of an opinion of it: speed, intensity, aggression, (sometimes, especially in its earlier years) a disdain for society's status quo and thusly a strong desire to encourage change and upheaval. Also, most of what helped it to become popular was based on the instantly accessible "three chord" approach which I've never been specifically fond of but it's obviously a tried-and-true formula for commercial success.
Blues:
Metalheads have historically been a subculture comprised mostly of social outcasts whom the popular kids would most commonly refer to as losers. In turn, we metalheads have found a common bond by cathartically venting our angst, sorrow, and despair through this counter-culture musical style.
Several decades before that, black folks in America endured a MUCH worse experience and their outlet was blues. Regardless of its redundancy or simplicity, its long term contribution to American music is undeniable. An overwhelming majority of rock and roll is based off of it as is metal. Beyond that, it still survives to this day as a well-sustained fringe genre that isn't a big player in the mainstream yet is avidly kept alive by many talented musicians just outside of the mainstream and it pops into the top 40 on rare occasions even years after its heyday has ended like with Tracy Chapman's Give Me One Reason (1995) and Kenny Wayne Shepherd's Blue on Black (1998.)
"pentatonic scale+power-chords rock":
This is just a big part of America's musical identity over the last forty years. It also reinforces my point about blues' credibility as a respectable genre since the power chords/pentatonic leads motif is quite similar to the barre chord/minor pentatonic thing that was a staple of blues for a while but this genre simply eliminates the 3rds (major or minor) from the chords and mixes that with faster leads, more energetic drumming, and an understood notion that it's okay to write songs that aren't centered on saddening issues. Beyond that, this style is more or less the connecting link between blues and metal and all three of these genres have been going fairly strong for 40+ years.
Conclusion:
Every genre has samey tendencies on one level or another so it's a very hair-splitting, piss match kinda thing to try to settle this debate in any remotely objective manner but I think a big part of the complaints toward post rock is that it's very drawn out in a lot of its songs and the payoff is frequently a loud crescendo of stuff that's a little less simple than the subtle, low-key stuff that led up to it. There's nothing wrong with that either but it's something that heavily goes against the grain in terms of what the average music listener is accustomed to.
It's kinda like going to a restaurant and being seated for 20-30 minutes as you smell all the wonderful flavors coming from the kitchen and then, once you're absolutely starving, the waiter brings you a single bite of food. Even if it's the most delicious bite of food you've ever had, the average person is gonna want more than that one bite and the common theme of post rock involving lots of build-up while not delivering something that's noticeably flashy or spectacular is kinda like that for some of us.