[2021] Why does everyone call the Frog zombie a spider?

Because they originally did climb up walls. But it was removed and replaced with the jumping system. Anyone who played in older alphas will remember them as wall climbing spider zombies.

Plus, in the game files, ingame questes, and on the wiki they're still called spider zombies.

Plus calling it the "Frogger zombie" really doesn't incite the same level of fear. God that scream they do still makes my heart skip a beat sometimes.

At one point, they were one of the most feared creatures in the game.

Good point, OP. Changing the name from “spider” zombie to “frog” zombie sounds like a good idea, assuming their behavior is decided. It’d be similar to the change from “feral” zombie to “wight” zombie (although that was because feral had multiple definitions in game, rather than it not fitting the behavior).