Maybe a better question is why broadband works at all, on a phone system that still uses Victorian cabling technology for the last mile.
If the job is done properly, there's no reason why the entire system cannot be weatherproofed. But in the rush to lay millions of miles of new competing cables over the last 10 years, I am sure some corners have been cut - especially in those countries that prefer lawyers over engineers. You can't be me, I'm taken