why is my game is deleting everything in ServerStorage?

This is because whenever you test out your game, you are testing on your client side. Whenever you are controlling your player in Studios, you are in the client side, which means you can only see what the client can see. This is to protect your game from exploiters by preventing anyone from just accessing all your code.

If you want to see what the server sees, press the run button(F8) rather than the play button.

It isn't deleted, it is just not visible. The client (you when testing) cannot see the ServerStorage. This is to prevent exploiters from accessing just about everything. If you look at the Top Bar, there's a button that says Current: Client. Click it and it will turn to Current: Server, then you can see what the server (you when editing) sees. This is also why local scripts cannot access ServerStorage. Only server scripts can see ServerStorage. This is the same with ServereScriptService btw.