why do people think its impossible to come back from a game?

Coming back from a game is only impossible if you have people who are actively trying to lose.

Nowadays all it takes is 1 screw up from the enemy team to get back into the game.

http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2385913674/47022294?tab=overview

This game I had 12.7k gold deficit and still won when nobody had full build.