Why don’t we start reading Parshas Bereshit in the beginning of the year-from Rosh Hashana?

There are other considerations involved, not just "having the beginning at the beginning". For example, we don't want to "dilute" either the seriousness of Rosh HaShana nor the happiness of finishing and beginning the Torah cycle, but rather designating and focusing on the 2 special and unique occasions seperately; we want to have the Simchat Torah celebration on Shmini Atzeret at the end of Sukkot, just as the "Atzeret" celebration ending the Pesach season is consumated with the giving of the Torah on Shavu'ot; we want to read the tshuva parsha in Vayelech adjacent to Rosh HaShana, and th "Eicha" pasuk (Dvarim 1, 12) adjacent to Tisha b'Av, and more.