Beacause 'The best manager in the world' isn't called God, he's called Pep Guardiola and his club have UEFA/G14 breathing down their necks waiting to pounce. PEG's ambitions and City's are different. PEG just wants more titles on his CV, but City are building a legacy.
Question: Would you rather quick fixes for titles now, or proper fixes for a long term legacy which meant few or no titles for the next 2-3 years?
Dunno about you mate, but I'm looking long term. I'm sick to the fucking back teeth of "Come back when you've got 20 of this and 3 of these'. We need to build a sustainable legacy which is something that both Mancini & Guardiola have pointed out. They've both said that it doesn't matter how much money we throw at our club, it helps, but it's not the be all and end all.
Clubs like ManUre, Real, Barca & Bayern have got decades on us in that respect and both Mancini & Pep have said it takes years to build up that kind of infrastructure and winning mentality from the Tea Lady to the First team & everyone in between. Money alone cannot achieve this, albeit with good management, it can accelerate the process.
We need to plan log term mate, not just for short term glory, hence the average age of the squad has fallen dramatically, not for our players resale value, but to buy top class young players who can grow up with the club, just as the European aristocrats have always done for decades. As it stands, we've only been doing this for the last 6-7 years and have a long way to go yet.