It's not laziness. It's so readers can figure out what the Kryptonese dialogue is saying, by replacing the letters. (The Kryptonese sentences are just English with the letters replaced.)
But pre-Crisis, the Kryptonese alphabet was more along the lines of what you're looking for, if you're into that sort of thing. It had 118 letters.
There's actually an interesting story behind it. There was a letter in a letter column asking DC to publish the Kryptonese alphabet. The response from E. Nelson Bridwell, DC's Superman continuity keeper, was that it'd be difficult, because the Kryptonese alphabet had 118 letters. Some time later, he actually set about creating a Kryptonese alphabet. However, Bridwell was *such* a stickler for continuity that he felt obligated to hew to his earlier letter column comment. And so it really did wind up with 118 separate letters.