How does a truck frame rust so much if it's painted at the factory?

Unless the truck is destined for California and requires the mandated emissions components the manufacture has no idea where the truck will end up, Could be Arizona for all they know, in which case no special care is taken to prohibit rust from road salting during the winter months in places like Maine, This would have been a dealership or other offered service in the way of undercoating, The manufacture wont do it simply because its an added cost and as I said they have no idea where the vehicle will end up, So whoever bough the vehicle was to cheap to have it undercoated or for all we know was from Arizona and just happened to move to Maine, not knowing they needed to have this truck undercoated or it would be a ball of rust in just a few short years, Paint is just paint, one little chip and here comes the rust when salt is added, And not everything is painted under your truck, and even if it is its minimal at best, and if you understood the physics of the whole thing you'd know that driving around at a good speed turns the undercarriage of your vehicle into a sandblaster removing that thin layer of paint in no time, Now that your truck has been diagnosed with Salt Rot the best option is to sell it if you can and make sure your next is undercoated or if new make sure you pay to have it done if living in an area that salts its roads in winter,

That paint they put on truck frames (and on cars when they had them) is to keep them pretty - - - - hopefully long enough for them to be sold. The metal is not properly prepped, and it's poor quality paint, poorly applied (usually by a supplier, not the truck maker), and thin......

The frame metal is thick enough to last "long enough" in the maker's opinion. If you're like me and don't like the idea of a rusty undercarriage, the questions is: do you coat/paint/rustproof the underside bits when the vehicle is brand new (and will whatever you put over the crap factory paint stick thru it) or wait until there is enough surface rust for something to penetrate and bind with it. I've had better luck waiting until the underside starts to rust, then coating with waxoyl, lindseed oil, non-detergent motor oil or such.

I am not sure how that frame was painted when new? Paint wears out over time.

Under a vehicle it is wet, cold, hot, chips from small pebbles/debris hitting it, subject to road salt and chemicals like concrete dust, etc.

So maybe if the previous owner had the car thoroughly cleaned and re-painted in say year 6, it might be no rust now. But how many people will spend the money to do that?

Was car properly at least rinsed off once a month, especially during winter?

1. It was likely not properly undercoated and undercoating has to be re-applied every few years (and people that only keep a vehicle for 4 years or less or that lease don't particularly give a crap about undercoating as it is destined to become somebody else's problem).

2. Maine probably uses a salt brine solution on the roads in the winter like the rest of New England plus if you have ever bothered to look at a map you will notice that large body of salt water we in these parts like to call the Atlantic Ocean in fairly close proximity to Maine which means that the vehicle is exposed to salt year round.

Why don't you send your never before heard of complaint (rust) to the manufacturer's suggestion box?

No you can blame most on Maine all the road chemical and winter salt eat the coating away. Down her in the lower Midwest ours all look kike new.