I think the key reason for those menus is so that content can be seen on the initial page load. This is similar to "above the fold" for a printed newspaper so it can attract the user's attention. Devoting that space to a navigation bar or header may mean that users leave the site or do not view the ads, which are missed opportunities for business. I see the hamburger menu not for convenient browsing around the site (I agree, it is not convenient in that regard), but as a way to save the initial space for what matters and will engage with the user.
Most of the time when I browse a mobile site, I'm coming from an external link and only visiting that one page, so seeing the content without having to scroll is reassuring that they care about the user experience. If I have to scroll through too much, I'm usually not going to continue. It isn't out of laziness, it is out of principle. They got me onto the page with the promise of some content that I'm interested in and then they are making me work to get it. It makes me question what their intentions are and what they are trying to sell me.
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