A lot of good points being made about this kind of bike's durability and function related short-cuts made to produce a $69 bicycle in the early 1970's.
I'll add to that the very limited frame size options offered on such bikes.

Also however we have to keep in perspective the eternal market demand for "disposable" bikes that involve a paltry investment.
Such bikes fill the needs of parents whose children may immediately leave their new bike unlocked, or who themselves may use such bikes as lock-and-leave transportation to a train, bus terminal or street-corner market.

The frames of such bikes were, until perhaps around 1990, typically hastily hand-brazed together, so without adding any material for a visible radius of brass. Spot-type resistance welding at select other points often completed the frame structure, not to be confused with Schwinn's own electroforging factory process.

Bikes at such price points have NO added anything that will add cost: no durability beyond what the market is deemed to require, no cosmetic finishing beyond what is needed to get the bike out the door of the store, and no such comfort enhancement as easy-to-modulate braking performance.
These are in today's market perhaps more like "3rd-world" merchandise when viewed by those more familiar with bike shops: they satisfy short-term needs of those in need of a very basic, limited-use bicycle.

Believe it or not, the makers of these bikes very much want to satisfy the requirements of the box stores that buy them, and the box stores in turn very much want to offer the customer as much in the way of perceived value as possible. That's all there is to it however, and aside from liability issues and also the inevitable product reviews that may turn up on a box store's website, no evidence of any passion for added quality is evident.

It's very easy to judge low-cost merchandise as unworthy when one has more money and product knowledge than does the intended customer of such products, but is also in some ways very unfair.

Schwinn took excellent, if heavy, "upright touring" 3-speed bikes and added derailer gears, leather saddle and a rather poor choice of 7/8" steel handlebar to create the Varsity, which the market demanded.
Others, like the builder of this bike, simply aimed for a less well-funded customer and produced a bike that they hoped would produce maximum sales volume and profit at the department store. It is what it is, and can be ridden gently for possibly a good long time if it is firstly checked over and then maintained in good condition.
Most consumers today would find no value in fixing up or maintaining this bike, unless money were very tight and there were no better choices in sight.
But it just might be perfect for use where theft of the bike or parts were a persistant likelihood.