And no one understands: why is this happening at all?

Owners of iPhones with SIM cards from the British telecommunications operator EE received blank SMS messages dating back to 2003.
As Metro writes , the appearance of such messages could be part of a malfunction in the operator’s networks or the operating mechanisms of the iMessage application itself. The second version is also confirmed by the fact that not all of the users who received the mysterious SMS use EE services.
The Next Web, citing one of the Reddit users, also suggested that sending empty messages could be a planned action of unknown hackers who, using a special SMS protocol mode, could collect information about mobile network subscribers.
None of the mobile operators whose users received suspicious notifications commented on the information that appeared in the media about “messages from the past.”