Like seriously, the cel era, hand-drawn animated cars look a lot better in motion. In newer works, I only see well-animated moving vehicles in movies or high-budget TV series.
I’m more generous in rating seasonal TV anime that have well-done vehicle animation.
In addition to what everyone else has already said: because unless you’re making an Initial D sequel, the cars are usually not all that important. Studios want to save their budget for the important stuff, so they aim for “barely adequate” on the cars and move on. (Honestly? I notice the horses more.)