Releasing published data

In my so-far short career in science, I have asked a grand total of four people to release their published data or a model that they had written. Of these four, only one gave me the data. Of the others, the one whom I asked for a model refused, saying that model was over 10 years old and he could not easily find the code. Another said the same for their data (which was published in 2002): lost, or hard to recover. A third did not answer the email (although they did answer another one, so my email probably did reach them), which I take to be a refusal.

I wonder if this is normal? Why do people not release their data? Why/how do they lose old data? It's a mystery that something so important as data, or a painstakingly built model, can get lost so easily.