Well, Half-Life 3 has never actually been officially announced, so it doesn't quite apply to it. But yes, there are always going to be exception. I think the biggest problem HL3 will have if it ever actually is released is that it's practically impossible for it to live up to the expectations fans have built up for it over the last 10+ years.
HL3 hasn't had a trailer, yes.
Valve has done all but officially announce the title.
It is not a title you can slap together.
All fans can really expect at this point is a title within the next ten years.
But, yeah it is an outlier; it is definitely not the norm.