There are a lot of client implementations that don't support this. You already can add a body to GETs but you'll likely run into issues somewhere due to this. I believe that JavaScript's fetch doesn't allow for this as one example. That's a big one that is probably important to have but there are many others.

Because a lot of existing proxies and other middleware will just drop the body. Or even worse not consider it in caching decisions but still proxy it.