Because the longer the series, the less chance for a weaker team to get lucky and fluke wins. This current Pakistan team can fluke a win over England in a one-off game, but they will never win a series if the respective strengths of the two teams remain.
Pakistan have been a pathetic, minnow level ODI team since for very long now, and that is why post 2005, Pakistan have played:
5 ODI series vs England, lost all 5
6 ODI series vs Australia, lost all 6
6 ODI series vs South Africa, lost 5
4 ODI series vs New Zealand, lost 3
The longer the series, the harder it is for a weak team to prevail.
When Pakistan won 4 games on the trot in CT17, ranked 7 vs higher ranked teams, that was a fluke according to you, yet here you are again undermining your argument again by proving the rankings are meaningless given Pakistan beat top ranked teams to win the CT17.
Sri Lanka was 7th at the time, and an equally shambolic side. Beating them was not an achievement.
However, Pakistan won one-off matches against South Africa, England and India. They caught them on their bad days.
If that Pakistan were to play that England, India or South Africa in a 5 match series in those conditions, Pakistan would have lost all three series.
The EPL ranking/league system is perfect and accurate measure of strength. Each team plays equal number of games, home and away.
That's how league football works, not sure why you had to highlight EPL specifically. Anyway, if that system is implemented in ODI cricket, you won't find Pakistan in the top three and neither would you find England and India at 6th or 7th.