It’s a combination of equipment setup and lack of training.
The headset provides very little side tone when making a PA due to the noise cancellation, so people put their finger over the back of the microphone and pull it close to their mouths. If they’re using the handset to make a PA, they’ll frequently hold it against their lips because they perceive themselves as not being heard.
You’ll get a PA that sounds acceptable by positioning the microphone about 1/2 inch from the corner of your mouth and speaking normally. Anything else sounds like you’re gargling the microphone.
A good airline will train pilots in the correct technique for making good PAs. This costs time, and time = money, so most don’t bother. It does have a noticeable impact on customer satisfaction so it’s worth it in the long run.
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