Because the videos show the issue with out requiring instructions to the user on how to repo the issue being addressed.

I've done this myself (usually with a gif though) because if I leave it as a exercise for the user to fiddle with 15% of them won't understand it.

I'm not sure it's worth caring about. Either way seems to communicate the problem perfectly. They provide a playground with code.

But my Canadian data rate...

Perhaps because 4% of user agents don't support it.

Thats not really an issue in this case. The only browsers not supporting it are ie11 and opera mini. (And a thing called QQbrowser which I confess I've never heard of.) I'm guess the 4% of traffic is largely ie11.

If you are browsing the Web today with ie11 then there's a lot of css that isn't going to work right.

And the impact of not supporting sticky is invisible - the table just works as it does now.

So there's no -techical- reason not to use it. At this point it's either a cosmetic choice (mostly for overflow reasons) or unawareness of its existence.

CSS sticky headings (the solution) won't work on the user agents that don't support it anyway, so they wouldn't be missing out. :)