Most people sell their new cars due to an expiring warranty period. Second most people sell their new cars due to the needs of a growing family. The third most people sell their new cars due to a changed financial situation. The fourth most people sell their new cars because they want even newer cars.
'Build quality dissatisfaction and reliability' is the fifth most common reason to sell a new car; or rather, to introduce a single-owner car to the used car market.
I had a JD Power infographic from ~2013 that listed the reasons, but am having trouble finding it.
Unless things have changed since 2013 signifigantly, most used cars that just exitted the factory warranty -- and sometimes factory maintenance umbrella -- have a lot of life left in them.
Aside from that, a car that is a few years older has a significant price advantage on parts -- a feature that is attractive to capable DIYers and mechanics.
And further aside from that -- if you're in the market for a used luxury brand car (Lexus/Infiniti/BMW/Audi/Mercedes/Porsche/ETC) , buying used allows one to side-step the dealer imposed major dealer-markup fees.
Example : the E92 M3 MSRP from from 2009 was about 70,900~ USD. The dealership markup in North Hollywood was 30.5k. In 2010 that same car was worth ~67,000 USD on the used market.
Many of these cars had less than 20,000 miles at the time of sale; buying a card with mileage that low is worth a 35,000 dollar discount to many people.
Anecdotally: bought a 2004 bmw from the original owner with 123k miles in 6 years. 7 years later. 185k miles, original clutch. (I'm also a mostly highway miles driver, but this also includes track days)
Most people leasing cars do it for fashion reasons, so they are normally getting rid of the car to upgrade to the next shiny thing, not due to problems.
Buying cars sucks and is entirely why Carvana and Carmax exist.
Also, if you can’t deal with the “pain” of buying a new car every few years you must find a lot of things really painful.