12V Car headlight halogens vs. 240V mains golf balls (longevity)
Dimming halogen lamps inhibits the halogen cycle that returns tungsten to the filament so you don't necessarily get the life extension you're expecting, especially if you run them many hours a day. You'll see the envelope getting dark prematurely as well. 

Your automotive bulbs are either on or off, so when they're on the envelope temperature rises to an appropriate level for the halogen cycle. 

Automotive operating duty cycles are usually very low in comparison to many other situations. 100,000 km on a car at an average of 45km/h is only 2200 hours. If you don't have daytime running lights the duty cycle will be lower again. 

I recall the halogen headlight bulbs in cars (eg. 9004) were quite poor in lifetime when daytime running lights were first mandated here in Canada back around 1990 (my 1991 car had the lights replaced several times, and almost exactly at the same time- the Japanese manufacturing was very precise) and they managed to improve the lifetime- and the bulbs lasted a reasonable length of time per pair. 

Presumably jurisdictions that didn't have daytime running lights (or used some kind of auxiliary lighting to meet the requirements) benefited greatly from whatever improvements they made.