Lintels in wood frame residential construction not always done by pros?

I've gone over BCBC 2018 revision 2 (latest as of writing) and I've read in detail about lintel (header) spans and sizes, from the span tables and the three sections that reference it. I can't seem to find anywhere that states it is allowed to frame an exterior door or window (or even interior non-loadbearing under truss roof systems) and in fact, even interior has it's own column....but here is what I've seen, locally and on youtube, of current construction by pros in Canada of doors and windows with no headers but just a 2x4 or 2x6 across the rough opening at the top with only cripples nailed to it. What am I missing here? Even 32" R.O. requires two 2x4's as a header.