Why si doesnot form double bond with oxygen?

Silicon DOES form double bonds. In fact, something like 70% of the Earth's crust is made of minerals containing double-bonded silicon... feldspar is the most abundant mineral in the Earth's crust, and it is mainly made of silicates. Silicates are silicon-oxygen compounds. Most commonly, silicon double bonded to two oxygens with another two single bonded oxygens (to make SiO4(2-)), but SiO2, silicon double bonded to 2 oxygens, is also fairly common.

The reason why there are fewer double bonded compounds with bigger elements (ie, further down the table) is because double bonds are shorter than single bonds, forcing the atoms' nucleii closer together. In the case of bigger atoms, the large nucleii make this difficult.