That’s because exFAT does not support any permissions of its own. It does not store any permissions, and as such the permissions are faked in the Linux virtual filesystem layer when the device is mounted, with an ownership and permissions mask that applies to the whole filesystem and that cannot be altered per individual file/folder, nor can the permissions be changed without remounting the filesystem.
ext4 on the other hand is a UNIX/POSIX filesystem, with full support for file ownership and permissions, and thus the permissions are the ones that were stored for each individual file/folder on the filesystem itself. 
Yes, but the problem there is that the mountpoint resides under /run, which is itself a tmpfs and which as such lives only in virtual memory while the machine is up and running. Ergo, changing the permissions on the mountpoint will only remain effective until the next reboot — unless one creates a udev rule that automatically sets up the permissions for that particular device to world-writable at mount time.