Why are weapons not called guns?

I'll pile on LTC Stephen F. 's comment. Besides the definition differences there's a significant cultural and performance aspect to "weapon". A weapon is something you must learn to be competent with and use only on proper orders and within the Rules of Engagement. We expect our MILs to know and properly maintain and use weapons appropriately. Don't get too wrapped around the axle on vessels and ships. Our new Zumwalt Destroyer is longer and heavier than the WW-2 cruisers and will actually fill the role of the old Battleship in the Naval Fire Support role which was reduced. So besides all the vertical launch and other weapons systems, it's two 155mm guns are equivalent to a battery of six Army 155s. Now don't get me going on frigates which were built larger than destroyers.....

SPC Margaret Higgins I do not consider your question to be naive.
In the military guns are sometimes associated with field guns which are a type of artillery.
There are weapons which use kinetic energy such as rifles, machine and sub-machine guns and pistols and revolvers. There are weapons which are held in the hand such as combat knives and sometimes bayonets.
Similar to the navy referring to vessels as either ships of vessels - they tend to cringe if you call a naval vessel a boat.

"Why are weapons not called guns?"
I truly do not understand what you're asking with this question..
However,
Simple Definition of weapon
: something (such as a gun, knife, club, or bomb) that is used for fighting or attacking someone or for defending yourself when someone is attacking you
If you are looking for a literal response, than, All guns CAN BE weapons, but not all guns ARE weapons.
If you're looking for something else out of this discussion you'll need to be a bit less cryptic general and limited in your question