Comment by jeffwass

Do you mean "why is there resistance"?

In 1900, 3 years after JJ Thomson discovered the electron, physicist P. Drude came up with a simple approach for modelling metals. He assumed electrons would scatter off the atomic ions, all behaving classically like balls. Even this incredibly simplistic model, which ignores the important effects of both the quantum nature of electrons, and the crystalline lattice ordering of atoms in a metal, can predict Ohm's Law. Ie, that current density is proportional to the applied electric field. https://en.m.wikipedia.org/wiki/Drude_model

That's probably the simplest explanation, and adding in more accurate assumptions to the model still shows that Ohm's Law still follows in the classical regime.

Though clearly the band gap (which involves insulators and semiconductors) has important effects when you look at the the impact of the atomic lattice on quantum states. And the 2nd-order electron-lattice-electron interaction that allows Cooper pairing and superconductivity.

You can also get strange things in nanoscale systems, such as non-Ohmic ballistic electronic current, where resistance is only due to the contacts of a device but not the length.