The design for the 737 cockpit is about 50 years old now, and it shows in that (as standard) is has no warning light for "cabin above 10,000 feet".
Now personally, I find it amazing that it was certified that way - but it was, and all the marks after are the same, due to grandfather rights, I guess. SO, just at a time when the pilots are hypoxic, they hear a horn which is only ever heard on the ground. Nothing lights up. That's one of the errors in the Helios accident.
Boeing (and the FAA) were faced with requiring the fitment of a "cabin above 10,000" light to the most popular airliner ever built, or adding a few lines to the latest manual revision. No surprise which action they took.