You can say that pretty much about any personal items -- laptop, phone, clothes, shoes, even your house. Manufacturing them cause harm to the environment and they're idle most of the time. Some of your clothes may stay unused for years, or forever. Yet nobody advocates for sharing them. That's because they are personal, reflect the owner's personality, and they're there when you need them.
Why is car different? I love train and tram (not so much bus) and would love to see them doing well, but I understand why people love car, and not only in less dense countries. Do you know that car ownership in Netherlands is on the rise? [0]. One of the reasons cited being people preferring cars as they get older.
If you want to advocate for public transportation, make sure you understand why people love cars, and strive to make public transportation suck less, primarily with flexible schedule, quicker, and more readily available.
[0] - https://www.researchgate.net/publication/358798695_The_wides...