>>96072154 (OP)>i just wanna make my own shitGood news! it's an rpg. THAT'S THE FUCKING POINT! you can even share these around with other people on the internet like imageboards. you should get to work and make this system and then share it with us instead of complaining.
seriously though, probably because a spell creation system that lets you produce spells that take into account the massive number of variables in a DnD spell would be insanely complex and difficult to make. (AoE, range, components, spell level, damage, secondary effects, conditional effects, damage type, can these be change when the spell is cast, what classes is it available to, casting speed/action type, ritual spell or not, spell school, specific use case or more general utility).
It's easier to just have a big list of a bunch of tested spells than make your own spell crafting system that could be "exploited" to produce ridiculous results like a 10 billion damage touch spell that requires skin-on-skin contact, somantic + verbal components, and an evil alignment called "Thousand years of death." especially since the point of the game isn't making spells but going on adventures, getting loot, and taking names. finding spells in said loot fits nicely into the intended design.