It's to prevent spam; there are people who literally let bots loose on the web just to find emails and put them on lists, then email spam/scams to those lists, sell those lists of addresses… It goes on. Generally just annoying email abuse. This [at] / [dot] isn't perfect, but it's a decent compromise between less spam and comprehendability on the end user's side.
But if you promise you're not a web crawling robot and you won't spam me, you can email me. Lemme know if you run into anything that's wrong and whatnot.
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