Just because Tesla has a recharging network and a car company doesn't mean others need a similar setup. It's fine for them to be separate companies. Just make sure your car supports ISO 15118 Plug and Charge.

It may be the case that it is easier to get a return on investment on installing chargers than building cars. If you go down that route, you might want to strike deals with car companies and their dealerships to include setting up your charger network on cars when they are sold.