My opinion: Because Apple didin't want to be a car manufacturer. It's a lowish margin and capital intensive business (especially compared to what Apple does). And just becoming one of many wouldn't actually move the needle on apples scale.

Most articles I've read focus on the electric car part, but imo that nowadays is essentially a solved problem. And even when they started I think it should have been clear that electric cars will actually have less complexity than cars with combustion engines. And the hardest part is the battery chemistry, which will in the end also be a commodity.

The general software they are already providing with Apple carplay and as seen this doesn't really require them to build cars.

The real technology problem to solve is autonomous driving. And it seems like Apple wasn't really able to solve it or at least make faster progress than others. Similar to Tesla which hasnt been able to deliver on that front either and is the only car manufacturer priced as a technology company. Which would have been Apple's goal.