Apparent quality is always of a subjective and transient nature. What Pele and Garrincha and Meazza did almost half a century ago might not be mind-blowingly incredible by today's standards. But we really need to look at it in isolation, and with relation to the footballing standard of that time. And try to project it against the comparable accomplishments of modern footballers vs their current peers. Sporting standards like almost everything else naturally scale in terms of quality with time, as we start learning from past players and improve up the trends they helped start, which along with advances in sports medicine/ conditioning, more detail oriented coaching/ dedicated training methodology from a young age, and a more comprehensive knowledge about the tactical nuances of the game gives younger footballers an inherent advantage.
To use a transferable analogy - bog standard physicists or mathematicians or cartographers today can replicate what Newton and Euler and Magellan did in their day. Hell, a lot of their work has been disproved or rendered redundant with the passage of time. But they will always be the gold standard for every present or future individual from that particular field of work, by virtue of their pioneering accomplishments that helped further the understanding of future generations.
Pele did a ton of impromptu things that no one had even thought of back in the day. From a very young age he set a new bar for athletic accomplishment and performance. Football as a whole took a leap with the advent of the likes of him, Di Stefano, Eusebio when we consider the amount of influence a striker or attacker could have on the game, they kind of even transcended the position to affect the general passage of play - unlike most strikers in the 40s and 50s whose sole purpose was to score goals.
You consider the longevity of Pele's career, the vast statistical returns, his performance on the biggest stages and the fact the he never really under performed in major tournaments - something that gets extremely underrated vs other pretenders to his crown.
eg. He scored 200+ goals in 195 games for Santos and Brazil vs European teams.
http://www.campeoesdofutebol.com.br/pele_jogos.html
Against the biggest continental rivals of Santos mostly in the Libertadores, his record was staggering:
Penarol: 9 games and 6 goals.
Colo Colo: 13 games and 10 goals.
Universidad de Chile: 6 games and 10 goals.
Universidad Catolica: 3 games and 7 goals.
Club Racing: 6 games and 5 goals.
America Mexico : 4 games and 9 goals.
Guadalajara: 6 games and 7 goals.
River Plate: 9 games and 7 goals.
Boca : 6 games and 5 goals.
Unlike Maradona who never consistently led Boca or Argentinos Jr to anything of significance, Pele's consistency was astounding - led Santos to 5 consecutive Sao Paolo and Toca championships. A feat that has never been repeated since. Maradona didn't perform exceptionally well in the Libertadores or the European Cup. But Pele was the decisive factor in Santos winning 2 continental and 2 intercontinental championships. Their win rate without him was ~35%, but with Pele it increased to ~80%. He never really failed at a World Cup in terms of on-field performance with 12 goals and 10 assists in the tournament - was great when fit in all of the World Cups he played in. In 1958 he scored the quarterfinal winner vs Wales, scored 3 in the semi final vs France of Fontaine and Kopa, scored 2 in the final vs Sweden at age 17. Got injured a in 1966. But in 1970 he reinvented himself to become a creative threat exhibiting the complete nature of his game. He was great in almost every season of club football - highest league scorer 11 times in Brazil. Someone like Diego didn't do a whole lot with Barcelona or Sevilla or Newel's Old Boys or Boca part II.
In terms of performance especially in the big matches, and athleticism Pele was ahead of almost every other player in that era. He could score in almost every imaginable manner - left foot, right foot, header, freekick, while dribbling, faux passing etc. The overall influence he had on games - even on Santos (+45 % differential when he played) and Brazil as an individual and how he inspired each team to a hitherto unknown level. Considering all of that, it's easy to see why he is considered the greatest footballer ever.