Application programming tends to be far more complex, far more planned, and far slower. Applications last for years and years, and new releases might take multiple years.
Whereas web programming can be extremely experimental, prototype-y, with multiple releases a day. It's also vastly faster to program.
This isn't an argument against good architecture -- far from it. But it does mean that the kind of long-term planning and investment that makes good traditional applications, isn't always applicable to website programming.
Due to the nature of the market, "complete disregard for maintainability or extendability" is sometimes a good thing, because you can iterate faster. Think of a lot of web programming as prototype programming -- then, you go back and re-architect, re-engineer, etc., as desired.