I think you misunderstood my question. I did not ask why Mirza did not "make" his shariah. I asked why he did not "combine" the four masalik. And if he did not combine them then which maslak did he follow.
I talked about Prophet of Islam making/creating shariat because chacha_ghalib asked which faqih did Prophet of Islam followed. I told him he did not follow any faqih because he was the creator of shariah (shariat-bearing prophet), but Mirza either had to combine the four masalik or follow some faqih because he was not the creator of shariah (non-shariat bearing prophet).
Prophet of Islam was giving laws, but Mirza did not give any new law (as per Ahmadis), rather he explained and rejuvenated Islam (as per Ahmadis). So comparison of Prophet of Islam with him is absurd.