The term "ma'aseh breishit" means, literally "the makings (or creations) of the beginning." The "beginning" seems to me, to refer to the first 6 days of creation.
Certain things that we recite "Oseh Ma'aseh Breishit" were not mentioned as having been created during the first 6 days, for example, comets and meteors (what we see as "shooting stars").
Lightning, in particular, is a curiosity. We associate lightning with thunderstorms. Genesis 2:5 says:
וְכֹ֣ל ׀ שִׂ֣יחַ הַשָּׂדֶ֗ה טֶ֚רֶם יִֽהְיֶ֣ה בָאָ֔רֶץ וְכָל־עֵ֥שֶׂב הַשָּׂדֶ֖ה טֶ֣רֶם יִצְמָ֑ח כִּי֩ לֹ֨א הִמְטִ֜יר יְהוָ֤ה אֱלֹהִים֙ עַל־הָאָ֔רֶץ וְאָדָ֣ם אַ֔יִן לַֽעֲבֹ֖ד אֶת־הָֽאֲדָמָֽה׃
No shrub of the field was yet in the earth, and no herb of the field had yet sprung up; for the LORD God had not caused it to rain upon the earth, and there was not a man to till the ground;
Usually, thunderstorms and lightning are associated with rain. While it is possible to have lightning without rain, and there might have been thunderclouds with no rain, it is unclear that either clouds or lightning were created during the first 6 days.
If so, why do we say "Oseh Ma'aseh Breishit" on these items?
I am making a parallel argument to this M.Y. question . One answer states that because the rainbow wasn't created during the first 6 days of creation, it gets a different bracha.