Wondering if it's more a UV ray issue which we can't see the damage done aside from fading. Car sitting outside for 2 years baking in the sun could in theory rot the belts; especially in desert southwest. Suspect 2 years is to CYA everyone. I'm in the aviation industry and we "time change" a multitude of parts that are perfectly serviceable all in the interest of public safety. The intent is to change or overhaul the parts before they enter a known window of potential failure. Over time (often in terms of years), these requirements are either shortened or lengthened as data is collected.

P.S. we do NOT change seat belts every 2 years on the aircraft i've worked on (visual inspection only)

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One of the members here some years ago did a pull
test on numerous old belt sets he had gathered, some
looked good, some were dirty BUT not a single set
broke the strap material... EVERYONE broke the hardware.
So just for kicks I did a couple of pull test on some
of my old sets at work... I wanted to see how the
failures were occurring on the hardware... as in
his testing, my testing showed the hardware failed
first...... but in ALL cases the force needed to
break the hardware was MUCH greater than the human
body can withstand