The maftir for many holidays, from the second sefer, has to do with the korban musaf of that day. But shabbat is a "holiday" and there is definitely a korban hayom as evidenced by its inclusion in our musaf davening. Why don't we take out 2 sifrei torah on each shabbat and have a maftir each week from "uvayom hashabbat"? [I was told that somewhere there might be a tosafot about this but I don't know where to look.]
Tosfiyos Megila 23a gives 3 reasons.
1- Since the Maftir of Shabbos has only 2 Pesukim and we do not read less than 3 Pesukim. We can not start from a earlier or finish with a later Pasuk as it has nothing to do with Shabbos.
2- The Haftora has to do with what we just finished reading, and if every Shabbos we read the same Maftir then all the Haftoras would be the same.
3- We only read Korbanos if they are brought as a Kapara, and the Korban of Shabbos is not brought as a Kapara.