Well, the reason that is because brave wants you to use that, same way mozilla wants to you use their account services and the cloudflare dns that you can opt out of but can't uninstall unless you use a fork of firefox.
And also I don't know why you put emphasis on space usage when firefox uses more resources on websites than brave and chromium, I tested it back when using xfce4 and for 3 tabs and the total system mem usage was 1.24 GIB for firefox and 1 GiB for Chromium and Brave. And when I did that test chromium hadn't implemented a new feature that they added that moves inactive tabs to the disk.
I also don't think I'm gaining anything by replying to your comment after you didn't even bother replying when I told you all the issues and missing features that firefox has and instead focus on hating brave. I hope you're doing this because you hate the brave ceo and not because of some weird fanboyism with firefox lol.