Hindsight is 20/20. Don't you remember what it was like the first time you started playing a game like this?
You learn that last hitting minions gives you gold during your first game, but that is not a priority. The first thing to figure out is how to not die every time you leave base. Your first assist gives you the taste of blood so you go for your own first bot kill, but they always seem get away with 50 hp. Every time you try to chase them down for the kill their turret just kills you first. You picked ignite/heal at the beginning of the game which would have secured the kill and saved your life but you always forget to use them.
Killing bots can't be that hard.. after all there always seems to be at least one person in your game who is 25/0/4 by 15 minutes.. but you don't know what "smurfing" means.
By around summoner level 20 you finally find a champ that you don't completely suck at. You've learned that tower diving is usually a bad thing. You're trying to last hit, but your lane partner always seems to be better at it than you are.. almost like they're deliberately timing their attacks perfectly to get it. You might have learned enough to be able to predict where the bot is going to go next so you're landing those skillshots more often.
Now you're level 30. You've got that shiny new Flash spell that seems to be everyones favorite, but you can't seem to use it efficiently, if you remember you have it at all. All that predicting ability you once had in those bot games is completely out the window now that you're playing normals. Everyone else is last hitting better than you... you don't know what all the champions do yet, so you're getting out-harassed. They tower dive you and you're 0/1 in the first 6 minutes of the game. You don't know much about junglers, you've never really needed wards before, so you push your lane too much and get ganked.. Heck even walking into your own jungle is enough to make your palms sweaty because your map awareness is complete ****.
Honestly, expecting people to be good at level 30 is a pretty tall order.