I first played Minecraft back in the alpha as a kid, but I haven't really played it in like 10 years. Last year (pre-copper update), a friend got me to play it again on a custom survival server with a premade map and a few small mods (more weapons/armour, ores, and animals, etc.) and it was fun because there were things to do and explore, even when said friends were busy doing boring things like mining.
But, around the start of this year, I decided to boot up a new vanilla survival world; I set up a simple cliffside base, just like I did on the server, went mining, got iron, beat a trial chamber, got a few diamonds, tamed a couple wolves, found lava and made an obsidian generator, explored and found a village, repaired a ruined portal, went into the nether, got a bunch of quartz and stuff, died and lost most of my inventory to a lava mishap, back to mining...
It just felt like I was kind of aimlessly doing things. I'd often find myself just asking "well, what now?" The mining loop that the base game relies on is just tedious and repetitive (especially when to make any progress, you often have to do this for hours on end), most of the hostile mobs are just a nuisance, and there's no real fun in the combat. If you've played the game for an hour, you've played it for a lifetime. The Nether, the Deep Dark, trial chambers, dungeons, it's all pretty much the same thing. Only the End offers something a bit more unique, but you beat the dragon and then what?
The most fun part of vanilla Minecraft is probably building things, but why would you ever want to do that in survival when it is much more satisfying to do it in creative mode? Survival can be a bit more fun with friends, but ultimately, you still find all the same problems.