That's because of a famous term coined as "Chalti ka naam gari", people tend to ignore these details and keep on driving, specially in the case of Beetles. Because as you know, these pretty tough things have a potential to run for years even without some parts that an average man thinks are not necessary. Only if it can run without it, doesn't mean one should throw such parts away. Designers and Engineers never put anything useless in a car. Why would they want to increase its weight and their cost? But people just do not understand these things.
Another very solid reason for a car with missing parts is, our local mechanics. They usually have fraudulent intentions to rip off some parts, specially from the cars that they know are quite rare and finding parts for them would be a hectic job for their owners, hence they can offer it to someone else and get some cash out of it. Sigh!
@farrukhahmed - I called a few times for that '64 Beetle yesterday but phone was not picked. Today he picked up but I came to know that the car has been sold about a week ago. Anyway I'm keeping an eye locally and if some Beetle comes up for sale I will let you know.