Train tickets now are often more than the cost of petrol to drive across the country. So if you'd spend more on a train ticket, why not drive?
Avoiding depreciation of the car or the cost of having one in the first place; lower risk of accidents; you get to use the time to work, read, or take a nap; you don't have to worry about parking. It's not all downsides.
My car's TCO is £0.64/mile. Easily beaten by the train.
Assuming I'd have a car already, though, I'd already have paid for the car, insurance, MOT, tax, blah blah. So for a journey I'm contemplating taking, where the choice is car vs train, the train is competing against the marginal per mile cost: roughly, fuel+repairs.
For this my car's cost has been £0.28/mile - a figure that usually turns out to be pretty close to the cost of the train. (In fact, this comparison is being unduly generous to the train, because most of my driving is short trips. For longer trips the car is more fuel efficient, making the marginal cost more like £0.20/mile.) I'm also not taking into account the cost of travel to the station.
But then again, I'm also assuming there will be free parking at your destination...
Just as an example, Bristol to Paddington is 118 miles.
118 * £0.28 = £33.04 for the marginal cost of driving.
An anytime ticket is £98.50.
An off-peak ticket is £43.00.
UK train travel seems to only be cheaper if you can buy an advance ticket (restricting you to the one particular train you've bought for), or with a season ticket (where the price is regulated).
Avoiding depreciation of the car or the cost of having one in the first place; lower risk of accidents; you get to use the time to work, read, or take a nap; you don't have to worry about parking. It's not all downsides.