Since they can't take any action on the notification and hence it's pointless clutter. Example:
https://en.wikipedia.org/w/index.php?title=User_talk:Koavf&diff=prev&oldid=1236492669
Description
Description
Related Objects
Related Objects
Event Timeline
Comment Actions
That example was also affected by T225009. But even if an indefinitely blocked user is the correct target there's still no point in notifying them.
Comment Actions
Leaning decline. Needs on-wiki consensus. This is frequently requested and frequently declined at the Twinkle talk page (example). Indefinitely blocked users or their talk page watchers may still be interested in XFD notifications.
Comment Actions
can't take any action on the notification
In addition to Novem's comment, they can still contact others via user talk page or email, if the user is not locked and talk page/email access is not revoked.