I don't think they have that much better squad than the others. It's just that Pep is a lot better manager than people give him credit for.
Barcelona won plenty of trophies before he arrived and plenty after (including the treble under Enrique). Bayern Munich won plenty of trophies before he arrived, plus the last 6 Bundesliga titles since he left. Man City had already started to win trophies before he arrived, including 2 Premier League titles, a League cup and an FA cup.
If anything, it's his predecessors that perhaps don't get enough credit - Rijkaard for giving debuts to Messi, Xavi, Iniesta, Busquets and Valdes, whilst integrating them into a side that contained players like Ronaldinho, Yaya Toure, Deco, and Eto'o. Rijkaard laid many of the foundations for what was to come (often with almost half of his squad coming from the academy), before he was unceremoniously sacked for a bad run just one season after winning the La Liga/Champions league double.
Guardiola won 3 titles at Bayern - just like virtually any other manager that is there for 3 or more seasons over the last 4 decades.
Mancini and Pelegrini won titles with City with far inferior squads, even though the big spending was already well underway at that point.
What Guaridola has done is spend over a £billion in only 5 years to turn the club into a flat track bully machine that no-one (except us with Klopp) had a hope in hell of competing with. There is no doubt Guardiola is an elite coach, but to say he isn't given enough credit is possibly due to some of the factors I've mentioned above - coupled with his arrogance and demeanor with the media.
There's no way for example, he takes a club a like Mainz and gets them promoted to the Bundesliga, or wins two titles with Dortmund in a one team league, or helps us to our first title in 30 years against the most financially doped club in sporting history. Those are achievements far more worthy of credit.