It makes it harder to vote, decreases participation, and the changes are not necessary in any way. You just don't like my reason. Plenty of states have been doing it this way for decades without any issue, including Washington where I used to live. It's only since Trump started screaming fraud that these things suddenly became controversial.

You're asking why we shouldn't change the voting laws to suit your preferences, which are in fact based on false understandings of the security of vote by mail. It is not banned almost everywhere in the world. It is legal in the UK, Spain, Germany, and Switzerland. I don't know where you got your claim that it is almost universally banned, but that's false (you could have guessed that because Trump made that claim and he says more factually incorrect things than any other politician in modern history). The link to the relevant facts is below. That link notes that John Lott was the source of the claim and he is wrong an astounding percentage of the time for someone who makes his living doing research. But he consistently gets cited by the right because the mistaken logic he uses in his research always benefits the same side of the political aisle. I'd call that suspicious, but I digress.

As far as the security goes, you may have come up with an argument for why it's insecure at some point, so you're welcome to point me to that if you like. I have never seen one that holds up to even a few minutes of scrutiny. As per usual, I'm providing links for my reasoning and you're asking me to do my own research to find whatever articles you claim support your side. My basis for saying that it is secure is two-fold:
1) Every study indicates that it is. See: https://www.brennancenter.org/our-work/research-reports/mail-ballot-security-features-primer or Google "security concerns with vote by mail" and you can find dozens of other articles.
2) Any efforts to conduct fraud at a large scale would inevitably break down just based on basic logic. Say you wanted to steal ballots from mailboxes of voters who hadn't voted in a long time but were still registered. You'd have to steal thousands of ballots, each of which has a security code on it to be traceable (you can't just stuff the ballot boxes). If even 5 percent of the people who haven't voted in a long time decide this is the year they're going to vote again, you're going to have dozens of cases of people complaining that their ballot was stolen. The same logic holds for why voter ID laws aren't going to prevent non-existent fraud. We know that reports of dead people or people who have moved voting almost always wind up being incorrect or one off cases, which don't scale to anything close to the level of changing elections (too small by a factor of 1000 or more). There's a more thorough logical argument on how this would be impossible here: https://www.cnet.com/news/politics/mail-in-voting-fraud-is-nearly-impossible-to-commit/ These efforts also break down based on how hard it is to keep a secret like that because of how many people would have to be in on the conspiracy. It's functionally impossible, as this breakdown notes: https://www.pbs.org/newshour/science/math-formula-charts-the-lifespan-of-hoaxes

Have a good night!