Because you shouldn't believe such things without evidence-
Reb Elchanan (Kovetz Shiurim II 30:1):
The Netziv (Meromei Sadeh beginning of Arvei Pesachim 99b):
I'm too lazy to find the Yerushalmi, but I think it's in one of the tosfosin on the first daf of Arvei Psachim. The idea is that Yerushalmi brings that the nusach of the bracha is מקדש ישראל והשבת, just like we say on Yomtov. On Yomtov, it makes sense, because our kiddush Hachodesh is a necessary ingredient in determining which day has Kedusha. The kedusha of Shabbos, on the other hand, seems to have nothing to do with our input. The language of the Yerushalmi's bracha indicates that we do contribute to the chalos din kedusha of Shabbos.
Reb Tzadok in his Pri Tzadik in Yisro:
Anyway, the point I'm making is this: It seems to me that the Rambam's idea of making kiddush Erev Shabbos only makes sense if Kiddush is statement of recognition, a ceremonial commemoration. If our saying Kiddush is supposed to make a chalos hekdesh, there is absolutely no way that you should be able to do it before Shabbos Mamosh. You can't be mekadiesh something before it exists.
It's kind of ironic that when Reb Elchonon says over the Rambam about making kiddush before you're mekabel Shabbos, as I brought above in the beginning, he quotes the passuk זכור את יום השבת without the last word- לקדשו. I think that Zachor, you can do on Friday. לקדשו, on the other hand, if you want to say that לקדשו means that you are making a chalos kedusha, that needs to be on Shabbos itself.
Reb Chaim B tells me that Rabbi Wahrman of HANC uses many of these Marei Mekomos and comes to exactly the opposite conclusion. I'm ok with that.