Americans making up the weird-ass fake name “Waldo” instead of using the real normal name “Wally” that the rest of the world uses is so strange

It's German. The early population of America was heavily German – with no overseas colonies of their own, the German diaspora (starting before German unification, of course) spread through the Americas – the thing about Nazis expatriating to Argentina postwar was that was the most advanced Germanophone community outside the WWII combatants – but a serious ethnosuppressionist campaign around entry to WWI has obscured this ever since.

You're saying that the name Waldo is German, and they called Wally that in the states because there are a lot of German-Americans? Huh, I had no idea.

Why not just call him Wally?

Oh, you mean the striped shirt "find him on this busy illustrated page" guy is called Wally elsewhere? I thought you were just dinging us for naming people Waldo in real life. No clue, I'm guessing there was already an American "Wally" of some sort to confuse the copyright. Also while it does exist in America, "Waldo" is rare enough that it's more memorable than Wally.

Curiously, American localisation of a certain similarly investigation-themed board game took the -do suffix off the end, rendering it ‘Clue’. This is probably explained by the fact the original name ‘Cluedo’ was a play on ‘Ludo’, which as I understand it is only really known in the States as the slight variant ‘Parcheesi’.