Because what javascript is changes rapidly and they always have to change to keep up and we all agree that that amount of work is difficult. So, parts of JS fail and don't run as expected over time even with constant mantainence.
This argument makes no sense, in that case the page would be broken for regular users all the same. Scrapers don't need to use some inferior browser and then pick up the pieces, they can literally use the same browser all the regular users do.
I think you'll find that people with complex screen reader setups don't chase the bleeding edge quite as much, in general.