Why change something that works - I respect the philisopy behind EndeavourOS - it aligns well with my perspective - but why change something that works for Manjaro just because EndeavourOS did?
The tools used to build the ISO is not designed for systemd-boot and would require quite some work to implement systemd boot.
systemd-boot is simple and thus limited in what it can do but you are free to switch - it is a fairly easy task
grub on the other hand has some features not easily achieved with systemd - most notably - IMO - the ability to boot an ISO stored on a local disk.