Because most people are right-handed so it's a good place to get their attention.

As far as I can tell the left & right bubbles UI was first seen on (or at least first popularized by) the SMS app on the original iPhone in 2007.

As another commenter suggested it seems the point was to associate it with the placement of the right-side enter button, as the message would animate up and to the right.

In RTL languages, it is flipped. And to answer your question, it is more about what feels intuitive, readable, and consistent with user expectations.

For languages that read left to right, placing your messages on the right creates a natural conversation flow — incoming on the left, outgoing on the right.

And consistency - it's a widespread design convention (used by apps like WhatsApp, iMessage, and Facebook Messenger), which builds user familiarity and reduces confusion.

I actually made a game with conversations recently. Originally it was flipped (player to the left). Something felt off so I got curious and realized the same thing you did.

The norm for comics is the person you are speaking to is on the right. Because dialogue is read from left to right and you don't want to cross the tails. The brain views the person on the right as being approached and the left as the one entering the room. It's inversed for manga. I actually started with artwork (first person, other character on the right). But I flipped it to match messaging norms on mobile.

A more obvious one if you play MOBAs is that your team is always at the bottom and the opponent is top. Yet the game is always able to reverse this for your opponent as well.

But most of the time when you open a messaging app, it's to read messages. Then the eye goes top left. There are few instances where the first thing you do is read your own messages. From a utilization perspective it makes sense too.

Fun fact though, a lot of UX is adopted from Japanese culture too. Like QRs and emojis. The western style is barcodes and smileys. So it could well be following the manga style.