Why is silicon oxide giant covalent structure?
Carbon dioxide is a molecule with double bonds.
Elements of 3+ rows are not happy with double bonds, they prefer single bonds, even in compounds like $\ce{H2SO4}$ 'double bonds' are actually dative single bonds.
The reason for it is that pi-orbitals for 3+ rows are very diffuse, making overall overlapping with p-orbitals of 2-rows extremely ineffective
For this reason, $\ce{SiO2}$ forms structure with only singular Si-O bonds. It happens so that for it the only way to form one is to make 3-d infinite structure.
For similar reason $\ce{N2O5}$ is exactly like this in gas phase while $\ce{P2O5}$ forms tetrahedral $\ce{P4O10}$  and $\ce{SO3}$ loves to form trimers and chains.