Because Arsenal will always be one of the teams to challenge the title. Its true that they've fallen behind and have been 3rd or 4th almost every season for the past 6+ years, but they've surely gotten better and have quality players like Alexis, Ozil, Santi, etc who can be creative enough. Now if you add an amazing goalkeeper who can rescue you around 12 points a season you are looking at finishing 2nd or maybe 1st if other results go your way. I think this season in the EPL will be great and competitive (I hope) with Chelsea United City and Arsenal all fighting to win the league. I really doubt Spurs or Liverpool will challenge the top 4 seriously.
they haven't bought any players expect goalkeeper the only team i think will challenge is man united