I think this is because its assumed you have your scores transferred in September when the applications come open. In order for a step 2 score to be included after that would require you to go into ERAS and "refresh" or "update" your score. This doesn't occur automatically from what I understand... In the grand scheme of things though, they're going to probably want to see your step 2 score before they rank you so you might as well update it.
edit to add from the eras guide:
The automatic retransmission is no longer an option when authorizing the release of the
USMLE transcript. Anytime an applicant has new scores available, they will have to manually
retransmit the transcript to programs to which they have applied using the retransmit
option on the checkbox will be grayed out.
