some have ansered this obvious question -and this answer may not serve for us now-
that since we review all the basic halachos in the yotzer we no no longer need to do so at the drash so the rav says some nice lomdus instead.
however the yotzer:
a) is not understood by everyone.
b) was written by a rishon but some of the halachos are not what we (especially ashkenazim but even sefardim) follow.
and last but most important, we need to hear halachos that apply to modern life like yes, vacuum cleaner bags. That is why many rabbonim now DO discuss halacha l’maasa.