Because it serializes the operation, whereas printing as you go operates in parallel.

But the modern thermal receipt printers are pretty fast (the old dot-matrix ones weren't). Epson has one that does 350 mm/sec (13.75 inches per second). So as long as you aren't shopping at CVS with their infamously long receipts, it's not too long of a wait.

I'm wondering as well. Perhaps the overall impact of needing to tear and reprint when there's an error that needs to be corrected?