In Frindle, why isn't it easy to do the research to answer Nick's question, "Where did all those words come from?"

This is essentially a large scale attempt at etymology, the study of words and their origins. In the novel Frindle, the main character makes up a new word and is at one point charged with discovering where words originate. The problem with language is that it is by no means static but instead is constantly evolving.
The origin of words typically spans thousands of years and multiple languages, especially with a dialect as complex as English (over one million unique, nontechnical words, compared to French or Spanish, each with between 200,000 and 300,000) and derived from multiple languages, such as French, German, Latin, and Greek, as well as now defunct languages such as Saxon or Celtic dialects. This is an immensely difficult task to track down words through the ages, especially as they themselves change, and new words are added almost daily (a quick search reveals words like “bling” in the Merriam-Webster dictionary, a word which would baffle people from the 1990’s alone).