Ever heard the saying "strike while the iron is hot"? Same thing here. If they waited, Marvel would be onto the next WORLD CHANGING NOTHING EVER THE $AME AGAIN!!!!! event six months down the line.
You basically answered your own question. It takes roughly six months to mass manufacture, print, package and ship large HCs with sewn bindings (usually from China). Marvel's own folks have commented to that fact. (I'm specifically recalling comments by Jeph York and others on the MMW MB). So they are working with a six month rolling window of finished product. In many cases, due to slow artists, the last comic book of that series may only be completed & released to the printer 1-2 weeks before it's released to comic shops. Therefore the quickest way to get a collected edition on the shelf, after the completion of the series, is to have it printed in the US with a glued binding.