It doesn’t, moving it to the end of the disk is a fairly common workaround for this specific issue. UEFI only looks for a GPT partition table and a partition within it with the UUID that corresponds to the EFI System Partition (ESP) type with a supported filesystem on it. The filesystem in question is implementation dependent, but FAT32 is guaranteed to be supported so most go with that. Apple’s firmwares can also do HFS+ (and APFS?). More advanced firmwares also let the user add their own drivers, in which case as long as you can find a driver for it you can use whatever filesystem you want.

It is common however to do so, out of convenience. Usually it’s other partitions you want to resize, and when imagine to a new bigger disk (or cloud environments where the disk can be any size and the OS resizes itself to fit on boot), then growing the OS partition is a lot easier. But the UEFI spec doesn’t care at all, some firmwares will even accept multiple ESPs on the same disk.