The installer gives the option to choose Openbox… The problem is: Openbox is too limited and hard to configure, it’s unusable for 99.99% of desktop users.
LXDE and LXQt are on top of Openbox, so if you install them, Openbox is there too, you can choose from the login screen if you want. They offer way more options, are extremely lightweight too and way more easier to use, it’s something a beginner could use if he/she doesn’t need a fancy desktop with lots of features.
In my opinion the Base and Openbox options should be removed, rolling release for server(Base) doesn’t make much sense and if you know how to use just the Base you probably know how to install Arch too… and Openbox is too minimal. If you go below Xfce in terms of RAM/CPU usage you loose too many features, there’s no need to go below it for most users. But that’s my opinion.
I don’t know the status of LXQt development, maybe it’s not ready yet, so LXDE is probably more stable and it could be a better option for now, and in the future move to LXQt.
Or remove Openbox and save some time of development and maintenance, it’s less code to worry about, less bugs, etc.