They're far simpler to manufacture once all the electronics have matured enough to be integrated. If you look at the Model 3, the power system is largely bundled with the battery. So the gigafactory is pumping out that module where much of the cost and complexity resides, and the rest of the car is largely devoid of precision machining processes.
ICE vehicles have complicated transmissions and engines, lots of machining and delicate assembly processes.
I expect there to be a period where the margins on EVs are quite fat thanks to inertia in the market's price expectations established by high-cost ICE vehicles. It'll wane once the competition matures and the undercutting commences. These cars should be much cheaper to manufacture (as well as operate/own/maintain).
My take away is that at the right price point, cheap enough labor and sufficient volumes the Tesla model 3 could have gross margins of 30%. Whether it is saleable at that price point and volume is unknown but more likely than the i3 and Bolt.