If you have enough time to average 50,000 miles a year...that traffic can't be
that bad..

However the national average that insurance rates are based on is 15-20k per year. So at that rate we're looking at 7-8+ years to get over 100k, and that is plenty of time for the rubber chassis parts to start drying out and making the handling start getting sloppy.
MilesAhead nailed it with the highway mileage being easier on a vehicle. Car lots frequently use this phenomenon to explain away high odometer readings on really clean newish vehicles by simply stating that they were a "Salesman's Car" ... Meaning that they were frequently (or exclusively) used for long trips so the mileage on the clock is not (to be perceived as) a detractor. Highway miles tend to get put on a vehicle much faster so the rubber parts are still fresh...because they haven't had time to age, dry out, and start cracking.
Hell, just look at the progression of vehicle inventory through various types of car lots to see what the real averages are:
0-30k - Dealerships never want to have anything on the lot over 30k because they like to stick to the cream on the top and auction off the rest.
20-40k - Reputable used car lots like vehicles in this range because they can still be sold high, and no major issues can be expected.
50-70k - Still safe to buy from low budget lots that tend to focus on offering "affordable" cars..
70-100k - Buy here/Pay here (because the bank won't touch it...), and the warranty if offered is extra...
100k+ - Happy Sam's (over)used auto emporium. These are cash only operations that offer in-house financing at loan shark rates. That will frequently sell and then repo a vehicle upwards of 17 times before they just give up and crushing the damn thing for scrap metal. These people are known for what is referred to as a "Tail Light" warranty ... The instant the tail lights disappear off in the distance (or around the corner)...you stuck with it..
Our company has several vehicles also (vans of various sorts), and they average 20-30k+ per year. However their usage is primarily in the city so the constant stop and bake, then go like hell (ab)use causes then to wear out quite quickly. We never keep a vehicle past 150k because by that point they are worn out to the point of being scary and way too expensive to fix.
One other side note is that the environment the vehicle is operated in can also be a major factor. The Florida sun is flat-out brutal. So regardless of which brand/quality of windshield wipers I get, I'll never get more than 3 month out of them before they get cooked out to the point of being dangerously ineffective.