They don't. The days when competitive / sportive cycling drove the designs of bikes sold to casual riders have faded into the past.
The most widely sold bikes in the US are "hybrids," mountain bikes, and cruisers. Within the mountain bike category, the most popular ones are relatively simplistic and are mainly used on pavement or light trails. Those are all relatively sturdy bikes with reasonably wide tires. Bikes that are specifically sold for "commuting" sell poorly, even among commuters, for reasons that I don't fully understand.
I live in a relatively bike friendly town, on a street that's designated as a bike thoroughfare, and I commute by bike myself, and I'm curious about bikes. So I make a mental note of what people are riding. The most common type of commuter bike is "whatever was in the garage," often left over from college or some past market fad. The people I talk to about cycling tend to do remarkably little maintenance. Most decent consumer bikes are sold with puncture resistant tires these days, which are a great improvement.
Now, from my observation (my own bikes, helping friends, volunteering for bike charity), bikes that go out of whack frequently, were usually built out of whack. Low end bikes that are shipped from the manufacturer to the retailer with little or no expert intervention often have poorly adjusted spokes and bearings, sometimes missing lubrication, sometimes parts installed backwards, and so forth. This is unacceptable, but it happens. An initial tune-up that includes bringing the spokes up to proper tension, and adjusting / lubricating the bearings, will turn many the crappy bike into an OK bike.
I don't believe a zero-maintenance bike exists yet, though a couple of mine are close. One is single speed with a coaster brake. I live in a relatively flat town. I also bought a new bike a few years ago for about 500 bucks, that has been maintenance free except for a puncture or two, and normal periodic adjustment of the brakes due to pad wear. It's a hybrid with 9 speeds, wide tires, and cable operated disc brakes. I love it.
But for this reason, I still recommend that a cyclist -- casual or enthusiast -- needs to develop basic maintenance skills. Simply being on the lookout for problems is often enough to catch problems before they progress beyond something just needing to be tightened.