That setting is set as default to give the highest level of compatibility to users who are not that advanced. Imagine that you are someone, who is not that tech-savy, as you. Now you want to install a messenger app from the app store and you are wondering why you don’t receive messages when you are not actively using the app. That’s a big hurdle for someone who isn’t an advanced Android user.
Furthermore, /e/ comes with several calendar and contacts synchronization services. These services wouldn’t work correctly directly from the start if you’d have to enable background data each time.
That is probably why background data is enabled by default. But I see no problem here if you are actually aware of what apps you install on your phone. You should always have the overview then which app can or can not use background data.