No, most aren't well built, but that's nothing specific to PCs or even computers. Most stuff is designed to be assembled easily, quickly and cheaply by unqualified workers. That doesn't per se contradict that the result is well built, but usually means that compromises are made to accomodate the assembly process, which diminishes other qualities of the product.

E.g. you'll never find a high-end CPU cooler in a factory-assembled PCs, simply because they are too difficult and slow to mount. Compared to a simple push-pin or lever-mounted cooler these can have a dozen parts or more and require funneling screws through cutouts in the cooler itself etc. -- you just won't see that on an assembly line.

There are also other issues, more specific to PCs, e.g. the standard case form factor does a poor job supplying graphics cards with fresh air. Other case form factors solve this (e.g. Silverstone has some; the "trash can" Mac Pro follows a similar concept), but tend to incur other compromises (and price tags).