I suspect it's because of the history of WSJT-X. In Meteor Scatter and some other VHF weak signal modes you keep the 1st report that you created for the station you are trying to work.
Back when people were using tape recorders and fast CW changing the report half way through a contact would be very confusing and potentially loose the contact.
Personally, I prefer keeping the original report. On CW I don't keep sending a new report during the contact either.
