As others have noted, there are several reasons why a linear regulator is not a good idea for a car charger.
It boils down to efficiency, which is horrible for linear regulators in general, especially when there's a large input-output voltage difference.
Add to this the cost to provide adequate heat-sinking when the current is above 100-200mA or so. Good heat sinks do cost quite a lot compared to other components.
Moreover, your linear regulator choice is also quite debatable: the 7805 chip is an old dinosaur. A much sturdier beast (almost as old, but much more rugged in my experience) is the LM317, as far as linear regulator goes. But still it can't handle more than 1A without external circuitry.
All in all switching power supplies are the right choice, and they can be quite rugged, if designed correctly, and still remain affordable.
Until now I more or less said what others have already said. However, what I want to point out specifically is the following statement of yours and some possibly related issues nobody has mentioned yet:
In past 10 years have tried more than a dozen chargers and they all break and stop working in a month.
Yes, indeed cars are a harsh environment for electronics connected to the car electrical system, but more than a charger a year is quite suspicious, especially if they lasted just a month or so.
If you purchased low-quality, el-cheapo brands this could be barely justifiable (I have bought some of them in the past, and none failed, although I admit I never used them for very long periods).
However, if you purchased quality ones I would start questioning your specific situation regarding "electrical cleanliness".
Maybe the electrical system of your car puts out particularly nasty spikes from the 12DC outlet, and this could cause the damage. If the outlet wiring was routed too near the high voltage circuits that power the spark-plugs, this could happen. You could search the Internet for your car model and see if this is a recurring problem.
Another issue could be ESD (Electro-Static Discharge) problems. Especially if you live in a very dry climate, the synthetic upholstery of the car, together with your clothes may act as a very high voltage generator (thousands of volts) when rubbing together. In turn this could cause yourself to being charged to a substantial electric potential.
Touching the charger while you are so charged-up could potentially damage the internal circuitry, especially if the case of the charger is light plastic and the circuit has little ESD protection. If you ordinarily experience your fingers being "zapped" when you enter or exit your car, that could be a hint of an ESD problem.
Of course the El-chepo charger brands are probably more susceptible to ESD damage, so the two issues may compound.