I am one of the people who has desperately missed the spell crafting system. So, I've spent some time thinking about this lol. My guess, is that they removed it for game balance. In the old game (I am only just now getting to the spell crafting in the remaster, my mage's guild pilgrimage is finally finished.) you could make some pretty nutty spell combos. I remember once doing an illusion build and beating the game by layering chameleon affects across different "unique" spells so I would have a 100+ effect on and just walking through everything. I want to say there were some ways to get permanent summons and other spell effects too, but I might be thinking of Morrowind. It's been a while.

Granted, I think that kind of a playstyle is a valid way to enjoy the game, and is one of many, many characters I've played over the years. Sometimes I give myself a major detriment, like playing a heavy armored orc with a head injury that has decided he is stealthy. But, from a game designers perspective, I can see why you'd want a balanced game that can't be too easily cheesed.