Thats stuffs only a problem because it slows the game down. It requires basically zero mental overhead.
Whereas I see it as 90% of the mental overhead for running a game, the part that's exhausting by the end of a session.Making stuff up on the fly on the other hand is low to zero effort.
I'm fine with it if it's making it up within a fairly tight framework, and the benefit largely outweighs the cost. For example figuring out a D&D 5e DC for a declared PC task is a relatively easy task after some pre-thought about how the numbers work, and has zero of the "slows down the game to look it up" problems that D&D 3e skill checks had. But as at least one poster have pointed out in the 5e forum over the years, there is still a mental overhead / exhaustion cost for having to wing them. (And it was only after that was pointed out to me to my initial total disbelief that making up stuff on the fly could be exhausting, that I payed attention to what it was that causes me to be exhausted by the end of a session.)