Use of integrals for multiple variables

Question

If we have more than one variable in an integral,why don't we always use more than one integral sign? For example if we deal with moment of inertia of a sphere,we take a volume element $dV$,then measure it's distance $r$ from $z$ axis and thereby determine the moment of inertia of that element as $\rho dVr^2$. For finding the total moment of inertia we then simply add all those of individual elements,which becomes $\int \rho r^2 dV$. But later on,we split this one integral into $3$ integrals for $r,\theta,\phi$ and hence we rewrite the integral as $\int \int \int \rho r^2 r^2 \sin \theta dr d\theta d\phi$. As we can see we are using $3$ integrals for $3$ different variables. Then why don't we do the same for line integrals? In line integrals,there is $x,y$ term present in a single integral sign. Shouldn't we use $2$ integrals in that scenario for those $2$ variables. This has been bugging me for a long time.