If you’ve had soups and broth made with lots of bones, and you want to recreate that same mouth feel and experience without using loads of bones, then you can achieve that by using gelatin, because gelatin is exactly what the first dish had that yours is missing. It’s literally the missing ingredient if you’re not cooking with the bones.

Also, they simply aren’t perfect replacements for each other. Agar and gelatin are certainly similar in many ways, but the are not the same.