Why do installers leave a gap between vinyl tiles?
No.   Professional installers do not lay vinyl tile squares like that.   These are literally the easiest tiles in the world to install.   These are butted right next to each other.   

The only reason they could be installed like this is the dumb*ss doing the install watched a tile install video and thought it applied to vinyl squares too.

Side note - there is really nothing you can do.   There is nothing that will fill the crack that will stick well and function in a kitchen.   You could poly over the whole floor but that could be creating more of a problem.