We need a sandbox mode!!!

why is there no offline mode in league of legends that can provide mechanichs-improvement and build-knowledge. I'm talking about a so called "sandbox"-mode with 0 cooldowns and 0 mana cost and infinite gold. In this way you can practise on many things without hurting anybody/ ruining someones game. As one of the biggest online games in the world league doesn't have this mode which is so incredibly weird. I've played several games, not even famous ones that have this mode and it really helps you improving and i could imagine this could be fixed easily, not much u have to change from the normal settings for the offline server. I'm pretty sure i'm speaking for a large group of summoners right now, and this should be added to league. Thanks in advance,