It made a big difference for me with occasional trips, I took public transportation to work, but Uber/Lyft meant I could go visit my friends for $10 and 10 minutes instead of $2 and 45 minutes.
I have multiple experiences with Toyota’s driven 200k to 300k miles with just standard repairs. A Corolla costs $20k? $25k?
Let’s say $1k for new tires/brakes/oil changes/filters every 45k miles.
That’s $333 per year for maintenance, and the car lasts 13 to 20 years at 15k miles per year for 200k to 300k miles, so let’s say $2k per year on the high end, $1,200 per year on the low end.
Extremely high liability insurance is $50 a month or less.
Fuel is 10 cents per mile if you assume $3 per gallon and 30 miles per gallon, so $1,500 per year in fuel for 15,000 miles.
$1,500+$50*12+$2k+$333 = $5k per year on the high end.
Excluding insurance and fuel costs, you can push a reliable Toyota/Honda’s costs down to 5 cents per mile or less.
Your maintenance cost seems optimistic to me, and you're also omitting parking and the cost of parking tickets (surprise yay), but besides that, I would say that there's a fairly high cost in terms of time and stress when driving a car.
As someone who doesn't own a car, I don't have to worry about parking tickets, or de-snowing my car, car not starting in winter. I also don't have to worry about finding parking, or parking tickets, dealing with repairs and scammy repairmen, paperwork, my car getting stolen or damaged, etc.
I also don't have to worry about driving drunk. It's pretty freeing to go out with friends and just have a car drop you at the door, and another car pick you back up when you want to go, and never having to worry about parking, when your parking timer might run out, or drunk driving.
And all of this being said, as someone living in a big city, between public transit, bike rentals, walking and ridesharing, the total cost I pay is less than your stated 400/month.
Others have commented on other points, but I'll note "Extremely high liability insurance is $50 a month or less." – the cheapest liability only insurance I can find is $120/month, and I'm 32 & married with a clean driving record. $100–$150/month seems to be the norm, talking to my friends.
I should have written "liability only insurance", meaning it protects me from everything except paying for my own vehicle, which I opted not to get since I can afford to replace or repair my vehicle out of pocket. I have $250k/$500k liability insurance incl uninsured and underinsured for ~$40/month from Amica on the west coast right now. I had it for $45/month or so from Geico on the east coast. Both my wife and I have clean driving record for past 15+ years.
I'm also talking about liability only insurance – from some other comments, it seems to vary a lot depending on exactly you're located. I did find much cheaper insurance when I was living in Maryland.
Different sources I find give between $1100 and $1600 as the annual average total auto insurance in the US, with about half of that liability. But there is a lot of regional variation. (state averages seem to vary by a factor greater than 2; and rates vary by ZIP code, not just state, and the range between ZIP codes has to be much greater than that between stats averages.) But, nationally, $120/mo. ($1,440/yr) for liability-only is very high.
AAA does this kind of calculation in more detail and publishes its results regularly for the average cost per mile driven for an average car owner in the US. It's on the order of about a dollar per mile, depending on the kind of car. Average sedan is about half a dollar, so you are still out by a factor of 10. It's obviously going to be higher in certain places too, like in cities, where there are extra and higher costs to car ownership.
Cars can be a great value, an expensive convenience, or a total burden depending on how these numbers actually work out and where you live and what you do. If you feel like a car is necessary, then the math probably makes sense. If you're at all hesitant, then the numbers would have to be very compelling to be at all compelling. Regardless, they only really work out if you actually have it for a very long time. If I wanted to purchase one for a year, and then resell, I'd lose a lot of money to taxes and depreciation, especially if it's new. Likewise, it's a huge liability that you just don't have if you don't have one.
Note that a car also costs thousands of dollars a year once you factor in everything, Edmunds estimates the true cost of a corolla at $7k/year: https://www.edmunds.com/toyota/corolla/2020/cost-to-own/