#5 It is a World Cup year
Everyone is aware of the talent pool Brazil always has, especially in offensive positions. Thus, it took a while for Coutinho to become a first-team regular. One can ask his Liverpool team-mate and no.9, Roberto Firmino, on the competition for places at Brazil as he is still in and out of the side.
Therefore, a switch to a club like Barcelona is risky with the World Cup looming in a year. His national team-mate Neymar plays in the exact same position as Coutinho's preferred one - the left wing. While Neymar leaving definitely gives Coutinho better first-team prospects at the Catalan club, there is no guarantee that Barcelona will not reinforce further in such a scenario.
Whereas at Liverpool, Coutinho is an assured starter when fit and can continue to maintain his form while improving his game. He can continue featuring for Brazil owing to the consistency and star in Russia next summer.
The presence of established superstars in Neymar, Iniesta and possible new signings Ousmane Dembele and Marco Verratti will lead to a tough competition. While there are good chances Coutinho will dominate the competition, the path to next year's World Cup is smoother at Merseyside.