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The short answer:
Because phone books.

The longer answer...

Where Did the Looney Tunes “Acme Corporation” Come From?

Acme isn’t a made-up word or a jokey acronym, but comes the Greek akme (the "highest point” or “peak,” and often used to mean the best of a category). Its heavy use in cartoons comes from its popularity as a name in the era when the Yellow Pages and other alphabetized phone directories became common.

We’ll turn it over to Chuck Jones, who worked as an animator, writer and director on Looney Tunes and Merrie Melodies cartoons for some 30 years, to explain it a little more. He said in interviews for the 2009 short documentary, Chuck Jones: Memories of Childhood:

“Since we had to search out our own entertainment, we devised our own fairy stories. If you wanted a bow and arrow, you got a stick. If you wanted to conduct an orchestra, you got a stick. If you wanted a duel, you used a stick. You couldn't go and buy one. That’s where the term acme came from. Whenever we played a game where we had a grocery store or something, we called it the ACME corporation. Why? Because in the yellow pages if you looked, say, under drugstores, you'd find the first one would be Acme Drugs. Why? Because “ac” was about as high as you could go. It means the best, the superlative.”