History. When mobile browsers first appeared they had to default to a desktop page width to avoid breaking exisiting content, much of which was either built in layout tables or used CSS that assumed a wider window. The viewport meta tag was introduced (by Apple in Mobile Safari when the iPhone first came out, IIRC) so developers could override that behaviour. I think the defaults made a lot of sense at the time, and Apple made the right choice.
<meta name="viewport" content="width=device-width, initial-scale=1.0">
> Why are mobile browsers defaults so wrong?
History. When mobile browsers first appeared they had to default to a desktop page width to avoid breaking exisiting content, much of which was either built in layout tables or used CSS that assumed a wider window. The viewport meta tag was introduced (by Apple in Mobile Safari when the iPhone first came out, IIRC) so developers could override that behaviour. I think the defaults made a lot of sense at the time, and Apple made the right choice.