Because the experts aren't really experts and peer review is useless? They routinely publish totally unscientific stuff and peer reviewers don't care, I can list 5 examples off the top of my head.
Why compare Germany and Sweden - because their case curves are virtually the same despite wildly different levels of masking. Because providing counter-examples is how you disprove a hypothesis.
Remember that he doesn't actually need to do lots of statistical analysis of this because mask theory is a total one. It doesn't come with caveats like "sometimes in some countries it works and others it doesn't for ", it doesn't even come with effectiveness numbers. It's just masks=less COVID. To disprove a theory this simple you only need one counter example. But this guy shows lots. He makes a big pile of graphs not because it's actually necessary to prove the point, but to try and ram it home to people who are in denial about it. There isn't just one counter example, there are a huge number of counter examples, and any one of them is sufficient to invalidate "masks = universal and effective COVID countermeasure" as a belief. That actually is science!
If mask mandate proponents were proposing a very subtle and complex effect where it was expected that sometimes there'd be no impact etc, then yes, the counter-argument would need to be correspondingly complex in return. That isn't the case here though.