Public transports are extensive, but they're overcrowded and perpetually strained to the limit of breakage. This week, a 400 tons concrete beam fell on RER C tracks. I suppose it will be weeks before it runs normally again. This sort of problems happens constantly.
Paris resident are only about 30% to own a car (a constantly dwindling number). Most cars in Paris come from the suburbs, therefore making motorists miserable isn't a problem for the mayor as most motorists vote elsewhere. This has the very unfortunate effect that nobody is really interested in solving the problem at its root.
At the moment, it costs a lot more inconvenience+money to use SNCF/RER than driving for many people.
If someone stabs you in the kidney, and someone stabs you in kidney and chest.
You don't thank the former.
Plenty of areas you can only move either with own car or calling taxi.
The next bus stop is a couple of kilometres down the road and only crosses a couple of times per day.
Yes, and it still sucks. Now imagine how much it sucks in other regions.