Why do sites do a back-button hijack if it’s possible to use the long-press menu-select feature?

Because it’s nowhere near equally convenient or standard to do so.

1. Long press doesn't show full history, it limits to 10-15(? Might be more). I've seen the occasional scummy background script that exploits this with numerous scripted redirects (usually pages with seo snippets cloned from SO). You have to disable the network to go back without being re-redirected, then go through a couple rounds of the back button history to get back to your original page.

2. If you're not using a full browser or are set up to keep a short history, you're basically SOL (Materialistic for HN has a built-in WebView, and I've used similar apps for other sites)

3. I shouldn't have to use history just because some site decided that it should add multiple history entries when I tried to scroll down an infinite list. I'm only on your website because it showed up in a search result, I'm not here to browse other unrelated content.