You say "they" like this isn't something that every single person in the world does constantly -- nobody is immune to it. Confirmation bias is just a derogatory term for people having priors.
Yeah, and it really is everyone. For example, remember when the White House Press Secretary tweeted a video that had apparently been doctored to make it look like a journalist had attacked one of their staffers? There was a really clear, convincing comparison that overlaid a semi-transparent version of that video on the original, showing that it leapt ahead in the part where his hand went near her because it had been sped up to make the interaction look more violent. Many people spread it, including YouTube debunker Captain Disillusion with this message: "Getting lots of requests on the topic, but there's nothing for me to examine. This person has examined it, very simply and clearly. And I concur." https://twitter.com/cdisillusion/status/1060564297103917056
Now, having compared the two videos myself and not found anything like this, obviously I had a look at it - and worked out the trick almost immediately. The comparison had the overlay ahead of the original by the same number of frames the whole way through from frame 1, but it was so faint that during normal viewing the difference was only visible in fast-moving parts, creating the illusion it leapt ahead in those parts. Easiest trick imaginable, could probably have been done trivially with film a century ago, and fooled someone who'd built a career and reputation around debunking faked videos along with many others.
(As for the White House video, that's a long story but the short version is that it seemed to be pretty much exactly what it purported to be, and the only actual "doctoring" was probably the obvious, intentional stuff like repeating parts of the video zoomed in.)