Q. When I light Shabbat candles I make a blessing. Why is there no b’rachah when I make my sukkah?
A. The question is what the mitzvah is.
In regard to the candles the mitzvah is to light them, so a b’rachah is necessary (however, the person or factory that makes the candles does not say a b’rachah).
With regard to Sukkot, the mitzvah is to dwell in the sukkah, and for that there is a b’rachah.
True, no-one can dwell in a sukkah unless and until the sukkah has been made, but making the sukkah is the preparatory stage for dwelling in it.