Because, effectively, the exFAT filesystem is OSS since August 2019 and Microsoft released all the specifications to it [0]. Microsoft has essentially taken public, legal steps to ensure that anyone else can use exFAT without legal encumbrances and without threat of lawsuit.

IANAL - so there's likely some caveats (you probably can't just use the exFAT name unless certain qualifications are met, just like you can't use Firefox's trademarks willy-nilly), but that should be good enough for just about anyone. Including paranoid OSS peeps like me :)

0: https://cloudblogs.microsoft.com/opensource/2019/08/28/exfat...