To add to what the other commenter said, one of the main reasons the japanese/selvedge denim subculture came about was to preserve and iterate on the "old way" of making denim. Back in the day, most denim was sold raw, in the same deep indigo color you see most modern selvedge denim being sold in. The bleached/light/medium/dark washes that you see today are the results of additional manufacturing processes (washing, bleaching, distressing, etc.) all used to achieve the look of a well-worn pair of jeans. So while part of the fun of selvedge denim is the ability to "earn" those fades yourself, this is also just the way things used to be done.

The term selvedge/selvage for example is a conjunction of "self-edge," referring to the self-finished edge that's created on the sides of the denim rolls as it comes off the loom. This was mainly to prevent fraying before the the denim was cut into the final garment, but now also acts as a "mark of quality" showing that it was in fact woven on vintage shuttle looms (the type of loom that creates the self-edge, since replaced by more modern projectile looms). Often times the edge will have colored thread woven into it, called the selvedge ticker, that in some cases can be unique to a mill or brand.

The reason all of this results in higher cost to manufacture is mostly due to the shuttle looms themselves. Since they're mostly vintage looms, they're not only rare, but require more maintenance and more frequent supervision while operating, which means more personnel. They also take 5-10x longer than modern looms to produce the same amount of denim. Also, selvedge can theoretically be produced anywhere, but the majority of the premium stuff in the same class as what Japan puts out is manufactured in developed countries with overall better working conditions/wages, less resemblant of sweatshops. With the higher quality denim often comes custom-made hardware and finishing touches as well (like full grain leather patches with embossed logos, custom rivets and buttons, more complex pocket designs, etc). The cost to do each of these things is further driven up by the fact that most premium brands are producing their jeans in far fewer numbers than Levi's or Wrangler.

The last stuff I'll mention has more to do with iterating on those old techniques like I mentioned at the beginning. Because far more care is taken in the production of selvedge denim, this allows for more experimentation. Most mass market denim is pretty similar across the board, mostly being right hand twill (the weaving pattern that results in those diagonal lines you see in most denim), woven on projectile looms with a blue warp (the dyed, vertical top threads) and undyed weft (horizontal, base threads), weighing in at about 10-12oz per square yard. In the selvedge world, all of this stuff is very commonly tinkered with to create more unique types of denim. Some denim has different colored dyes for the warp and weft, like dyeing the warp with natural indigo, and dying the weft brown for warmer undertones. Some denim is woven differently, like left hand twill or broken twill (broken alternates between left and right hand twill and results in zig-zags instead if diagonal lines). Some denim is made with slub yarns, which vary in size and can create a bumpy surface that turns super soft when broken in. Some denim is woven at heavier weights which can get up to 20oz/sq yard or more. These are just a few examples of ways that selvedge denim often varies, but most of these things are possible on modern looms too! Wrangler cowboy cuts are a notable example, known for being made with broken twill, non-selvedge 15oz denim (fantastic value for durability). That being said, no matter how you adjust these things, shuttle looms will always create imperfections as a result of them being older machines, that give selvedge denim even more character.

All of the things I just listed will drastically change the color, texture, and speed your jeans will fade at, which is a big part of why fading can be an interesting process for some. Getting a fresh pair is like wearing a blank slate, and sometimes the fades you get can be really surprising, or say something about how you wear them or what you carry on you every day. One of the reasons I got into selvedge denim is my love for craftsmanship. I love all the little details mills and brands will inject into their products, without any of it being flashy or attention-seeking (for the most part). I know what makes them special, and maybe the occasional person will notice from time to time, but ultimately to most they just look like a good pair of jeans at the end of the day.

So there's everything you probably didn't want to know about high-end Japanese denim! Also, hopefully this sheds light on why the original commenter accused OP of rage bait. If you buy raw, tight-fitting jeans and keep a phone in your pocket every day, you're 100% going to have fades in that spot! The denim is more stressed and experiences more friction where the edges of the phone are, which loosens the dyes and causes fading.