Because you can support Wayland and make Sway better. X11 is a super crusty, hacky mess that is being phased out. Sway also incorporates some nice features (gaps) that were previously not in the base version.
I really don't get it why people like gaps so much, yes it's look pretty, but in my opinion it is as much waste of space as window drop shadow. I would like to have as much screen real estate as possible to be used to show information, that's the whole point to use tiling wm in the first place for me.
A 5px gap between windows is a drop in the bucket, but adds a lot in terms of aesthetics.