Why can't I ask Siri to convert time from 24 hours to 12 hours like I can with measurements?

Ask anyone in the military or who has served in the military, which is what a 24 hour clock is, if they say 22 o'clock and they'll laugh and tell you no, they don't. The correct way to say what you wanted to say is 22 hundred hours. And if you do so, Siri will give you the answer you want. If you don't say it correctly, Siri won't.

And Siri just gave me the option to use ChatGPT, which I chose, but wasn't required.

If you want to let Apple know you think Siri got it wrong, feel free to do so. You most assuredly haven't done so here. This is a user to user only forum, NOT Apple Support. You can provide feedback to Apple, but don't expect a reply and as I've already told you, 22 o'clock isn't correct. See the Product Feedback Link below.

The following details how a 24 hour clock would announce hours:

Saying Hours in Military Time

In military 24-hour time, hours are typically read and spoken in a straightforward manner, with each hour and minute clearly articulated. Here are some key points to help you pronounce military time:

• Hours from 00 to 12 are pronounced with a leading “zero” for the first digit, followed by the second digit. For example, 0600 is pronounced “zero six hundred” or “oh six hundred.”
• Hours from 13 to 23 are pronounced as they are, with the tens and units digits spoken separately. For example, 1700 is pronounced “seventeen hundred.”
• Midnight has two designations: 0000, which is pronounced “zero hundred” or “oh hundred,” and 2400, which is pronounced “twenty-four hundred.”

Product Feedback - Apple

For one, one wouldn't ask for 22 o'clock. One would ask Siri to convert 22 hundred hours into 12 hour time. Siri, then asked if I wanted to use ChatGPT, which I said yes to and it told me the correct answer, which is 10:00 PM.