If it's good research, it's more likely to be published in an academic venue (and possibly the other way around). So if there are a lot of papers, I will prefer the academically published ones.
I have limited time to read publications, I am not going to read everything just because it is available somewhere. For me, the amount of papers published these days is an argument for academic venues, not against them.
(As I am a computer scientist I have a hard time writing "journals" instead of any more general term, as conferences are way too popular in this field.)
You're outsourcing the QC to a trusted third party.
Reading papers takes up enough time and effort as it is. I do not have the time to reproduce all of them.
I shouldn't have mentioned snobbiness; people've gotten caught up on the wrong part of the message.
For me, trustworthiness. Not all journals are equal but some are held in such high esteem that I would grant a lot more credence to the findings of an article published in a journal than one self-published.
I have neither the time or will to assess the merits of each individual who might publish something. If a name is sufficiently big within a field, it's usually in a sufficiently big journal; on the other hand, I would treat a self-published article with the same level of skepticism as those low-tier journals that happily publish pseudoscience as fact.
Call it snobbishness if you will, and I'm fully aware that academia is full of it, but that's the role that a journal fills for me.