I can think of a few reasons that I would not buy a flash like that:
First, the internal battery would degrade over some years and then you have a useless flash. All my current flashes are at least 20-30 years old and are running fine with modern rechargeable AA and AAA batteries. Also, the nice thing about using replaceable batteries is you can take advantage of new technology as batteries improve instead of being stuck with what is in your flash.
Also, what if you drain the battery while shooting or accidentally leave the flash on and it's dead when you want to shoot? What do you do? You can't just swap in fresh batteries and start shooting. You have to plug it in and wait for it to charge.