Super intelligence isn't always something he has. It varies between writers and editorial direction.

That being said, magic in DC isn't something you learn purely by brain power. It isn't just reading a book and knowing the right words. Even Zatanna's magic form isn't just saying backwards words and it happens. There's an underlying process of applying/channelling magic that takes practice to get right.

So, while supes could likely grasp the principles of it, he would still have to spend the time mastering the nuances. Well, until and unless the editor/writer needs him to ignore established magic in universe for a specific storyline. They could grant him super-insertabilityhere just like has happened with most of his power over the decades.

But, if you look at the "bigger" magicians in DC, there's always some degree of subtlety needed that isn't going to be bypassed by super speed or super intelligence.

There's also the aspect that there have been hints that DC treats magic as an inborn ability that you have to have to be able to learn to use it.

There's certainly a difference in magic ability that isn't reliant on reading and thought alone.

I mean, the real answer is that DC knows supes is over powered already, and there's a limit to what else they can add to him without totally breaking credibility. But, in universe, he hasn't shown any ability to do it.