The way MMORPGs implement it, crafting is dull, tedious, and limited. The players can't employ imagination to create novel items, they are restricted to a small set of fixed recipes. The game designers couldn't allow much latitude on crafting, or the players could and would make items so powerful that they completely unbalance the main game.
It's not just game balance, it's coding for the flexibility. It's one thing to code eight potions for eight reagents, with the restriction that each potion gets exactly one reagent, and another thing to code for roughly 100 different potions, if you free it up to allow each potion to have any combination of 1-8 reagents in it. What do all those potions do? What are they called? Can you even think up 100 different unique effects for them to have? And, ultimately, does it even add anything to the game to have a 100-potion exploration space (and do you give hints for recipes, or let players keep a cookbook to remember, or is discovery part of the game?) rather than just a simple setup of 8 obviously handy potions. Is the time required to allow this advanced crafting better spent on coding more spells, or another level, or doing more gameplay testing?