Why is CH4 Nonpolar but CH2Cl2 is polar?

I keep reading that the reason why $\ce{CH2Cl2}$ is polar because due to its tetrahedral shape, the dipoles can not cancel each other out but doesn't $\ce{CH4}$ also have tetrahedral shape too? I assumed, since the reason for $\ce{CH2Cl2}$ being polar is apparently due to the 109.5° angles from the tetrahedral shape, that this means that the surrounding atoms in all tetrahedral-shaped compounds are not placed directly opposite of each other and that is why the dipoles can't cancel each other out. Am I understanding this wrong? Or if this is the case, then shouldn't the dipoles of the $\ce{H}$ atoms in $\ce{CH4}$ not be able to cancel each other out either because they aren't directly opposite of each other? Is it different because all the surrounding atoms in this case would have the same dipole?