As the overall percentage of site owners nowadays are probably non-developers, they might simply want to get their WordPress site up and running by themselves, in as little time as possible. Which is exactly what ThemeForest themes (often) offer and deliver. If not, you "wasted" $50, maybe you can even get a refund. If not, stop crying and move on. Still less damage than to contract a dev/agency to develop on your new site from scratch, which you end up not liking.

You can think about Avada whatever you want, but I look at the numbers and see a 2.5 year old theme, that is in active development, has 9.320 ratings with an average of 4.75/5 stars. And many Avada customers have probably switched themes after Avada. Thousands of them could have given a 1 star rating because of data portability issues. Yet the overall rating tells a different story.

Yes, data portability is important, but if all the average theme customer can and wants to do is to upload a theme zip file, and is not concerned about (or aware of) data portability, than don't blame the theme dev all day long for the fact, that (s)he is actually giving the customer what (s)he is asking for.

After all: if getting your site, and for many people its their very first site, up and running, is the most important factor, when deciding which theme to purchase, then I can only quote Louis Reingold:

I care what the USER thinks a theme should be. Don’t like “everything” themes? Don’t use them!

Having a "100% Portable" badge or something like this next to each ThemeForest WP theme would at least help people, who consider this an important factor, to find such themes.