Syntactic sugar that's only natively supported in PostgreSQL? The site's banner says, "A lot has changed since SQL-92". Be that as it may, but it seems no one has really bothered catching up. I wonder why that is...

My guess is that such extensions, while useful, are somewhat marginalised features in terms of usage. Thus, no one ever learns them formally and just Googles for what they need -- if it comes up -- and get the CASE solution, in this case (pun unintentional). Hence perpetuating that pattern. Also, of course, the CASE solution is a lot more powerful as the returned expression, that gets fed into the aggregate function, can be basically anything.