As a handout to starting players, sure. But suppose you are a keeper that wants to set a scenario during the Chaco War and you want period accurate and realistic weapons for both sides, up to and including things like a Vickers tank. With your two page supplement as your only guideline, you are going to be going to wikipedia and then doing your best to translate the information you find there into rules hoping that your knowledge of guns is enough that you are making reasonable rules. This process will take you hours of your time. If some professional has done this work for you, you just flip to the appropriate page in the weapons tome and you spend your time actually writing your scenario rather than doing research and rules smithing.
And note, "just use wikipedia" is itself a very modern solution. In the past this required dozens of hours of research in college and public libraries.