Why Are Words Like "The" And "It" In The Dictionary?

Re. the above:

The O.E.D. was started in 1857. Its first edition was completed in 1928. Its second edition was completed in 1989.

The O.E.D. only covers modern English. Its companion dictionary for Middle English was started in 1927 and is only now approaching publication of its first edition.

In terms of ongoing committment and multi-generational effort, these dictionaries are the modern day equivalents of mediaeval cathedral projects.

Not including words such as “it” or “the” would be no more than building a cathedral without foundation stones.

The OP has a 16th-century sensibility as to what a dictionary should be. The original English lexicography in those days only included the “hard” words. Reading was considered a gentleman’s occupation then, don’t you know. Why, the very thought that a proper Tudor gentleman would ever need to look up “easy” (common) words! At least that was what they thought at first. It wasn’t until Dr. Johnson in the 18th century that English lexicography began to include all classes of words.