Halogen bulbs do dim out as they age. The most common problem with modern headlights is the plastic covers get oxidized (cloudy) due to sun exposure. The cloudyness blocks some of the light from the bulbs, and scatters the rest.

https://www.theautodoc.net/blog/why-do-headlights-become-clo...

The other problem is that some bulbs are just not very good - the filaments aren't properly positioned, or they don't have a good spectral output.

3M has a kit for polishing the haze off headlights with a drill, and for restoring the UV protection layer [0]. Dan Stern [1] told me it's really better to just get a new OEM headlight.

[0] https://www.amazon.com/3M-39008-Headlight-Restoration-System...

[1] https://www.danielsternlighting.com/

I suspect I had a wiring issue that was causing the lamps to burn out prematurely, either that or the lens housing wasn’t watertight and moisture/condensation was destroying the lamps.