Why do the LotR movies downplay Gandalf’s powers so much?

In the Books, Gandalf could change into a shadow form which gave the appearance of being larger than he really was. He could turn invisible, just like the One Ring Bearer, as he did in the Goblin King's realm in the Hobbit. He could throw fireballs and lightning, and deflect fireballs and lightning and he could cast locking and unlocking spells which could only be broken by other super-natural beings. Although he was mostly limited to the mortal form of a human being in terms of speed and physical strength, Gandalf was nevertheless an immediate master at wielding Glamdring, and he even defeated one of Sauron's Captains in a single blow at the gates of Moria near the last battle in the campaign against Sauron.

In the movies, you just see Gandalf produce a beam of light which apparently blinds the goblins and other foes. The only bolt of lightning he used, I think, was against the Balrog, but in the books he actually fried the Nazgul several times during several fights. They just eventually regenerated and came back from it. Once was at Weathertop hill, and another time was I think at Rohan or one of the other cities, I forget. Anyway, he hit the Nazgul in-flight with lightning and knocked it down.

Also, at Rivendale, Gandalf and Elrond (apparently) control water and turn it into a wave and horses made of water and trample the Ring Wraiths. They did a poor job of this in the movies, in my opinion.

How would Gandalf's battle with the Balrog stack up compared to other fictionalized battles, such as Cloud vs Sephiroth in Final Fantasy Advent Children?

Why do the LotR movies downplay Gandalf’s powers so much?
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