@Leonkristoffer because some random mangaka came up with this, put it in his trash manga and surprisingly people bought it and he made a profit after which every other mangaka just copied his whole manga word for word (the decent bits and the shite bits indiscriminately) because why make something new when you can plagiarize and get free monies.
This is why 90% of isekai have appraisal, dimensional storage, the same medieval Europe setting, the same monsters (goblins, slimes, demons), demon lords, the same kirito face/hair design, rpg stats, brain dead nobles who are cartoonishly evil, the same 4 core races (elf, human, beastman, dwarf) (when i saw interspecies reviewer i was astounded by the diversity of races, tfw softcore hentai is more creative than mainstream fantasy manga), harems, female character either have G's or A's no in between etc etc etc.
isekais do not take kindly to new ideas, even the tiniest, slightest change to the formula is frowned upon among the mangaka circles i guess. Ever seen an isekai where the mc/other adventurer parties use horses instead of walking across the country to do a quest? ive seen maybe one or two, you'd think something as simple as a horse would be just as common as appraisal skill but nah it's too far outside of the comfort zone of the lazy dip shit authors.