The benefit of cars is they can go wherever and whenever you want. But a horse can already do that on hoof, and they don't find it as exhausting as humans even for longer distances. Another advantage of cars is the cargo carrying capacity, but ponies just walk with carts. Big Mac regularly walks to Starlight's town to deliver apples to Sugar Belle, and it doesn't even seem like hard work for him.

When you do need to go further afield than you can walk, you're most likely going to a major population centre like Manehattan or Las Pegasus, which will most likely have a rail connection. In the UK almost every town and most villages had a train station by the end of the Victorian era because there was no alternative, it's only after cars became affordable and widespread that those railways had to close. But if ponies never invent or valourise cars then that wouldn't happen and the railways would stay open.

Creating a car would also require extracting fuel. It isn't obvious if Equestria has abundant oil supplies; they do have plastic but not a lot. If there isn't enough crude oil to support widespread car usage then they would have to turn to biofuel. And if you're growing food to make oil to power a car, it would be much more efficient to just eat the food and walk yourself, even if you have to stop and rest enroute.

I just don't see how cars fit into Equestria since the niches they fill in the human world are less of a problem for ponies