Apart from the ecological fallacy in your premise, which was pointed out in other answers:

Blog posts and lecture slides don't carry the scientific authority of peer-reviewed and professionally edited publications like journals. Peer-review and the services of scientific editors serve as a vetting process to filter out publications that don't hold up to scientific scrutiny. The vetting is performed by (usually two or three) qualified researchers on behalf of the scientific community. By contrast, the credibility of a blog post or lecture slide depends entirely on the reputation and trustworthiness of its author. Not only is this a less stringent criterion, it is also hard to assess for outsiders.

Apart from the overgeneralization in your premise, which was pointed out in other answers:

Blog posts and lecture slides don't carry the scientific authority of peer-reviewed and professionally edited publications like journals. Peer-review and the services of scientific editors serve as a vetting process to filter out publications that don't hold up to scientific scrutiny. The vetting is performed by (usually two or three) qualified researchers on behalf of the scientific community. By contrast, the credibility of a blog post or lecture slide depends entirely on the reputation and trustworthiness of its author. Not only is this a less stringent criterion, it is also hard to assess for outsiders.