You have limited number of goalkeepers in the starting eleven (one). Top 5 is world class.
Handanovic is top keeper the same way as Dybala, Lautaro, Osihmen are top strikers.
Actually the first factor makes the second not so relevant. There can only be one goalkeeper at team, so there's more parity in them. There are teams with very good goalkpeers than no one sleeps on. In this day and age, the top players are typically gathered in a handful of clubs, so you can see clubs with 2-3 top midfielders etc, but other than PSG, no one has more than 1 top GK. PSG has 2 + a possible starter for any top half team in the world.
Emiliano Martinez is one, although he's become rather shaky with fame and the burder of #1 Argentina keeper. Especially since he's not better than Musso who's #2 now, and Juan Musso is another one.
Top teams don't always have the absolute best GK.
Ederson is only great playing from the back, he's slightly better than Onana overall, but he gets hyped because City fixed their defense and either way, almost nobody has the ball on their side of the pitch. He's the best fit for them, not the best possible goalkeeper.
Alisson is far better than any Liverpool GK ever, but he's not that much better than Handanovic and is more prone to errors.
I don't get the Mendy hype, there wasn't even one in France. He's just a good shot stopper, there are dozens like him. Onana being one as well and I reckon he's actually better.
Szczesny is bang average. [can't believe I have autocorrect for his name
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Ospina is good but not Handanovic good.
Maignan isn't a top 3 Serie A keeper and that's what he was called to replace. The gap between him and Tatarusanu is smaller than the gap between him and Handanovic.
PSG alone has two goalkeepers that are better than the ones mentioned above. And their 3rd, Sergio Rico, is on the same level as half of those.
De Gea has his great moments but he's way overrated due to the Manchester United popularity situation.
Pickford is one of the PL's best keepers, although quite inconsistent, and he's not someone many top half Serie A teams would trust.
The absolute best goalkeepers in the world right now are Ter Stegen, Donnarumma, Oblak and still Neuer. The next tier includes Handanovic who remains one of the top keepers. It doesn't matter if this list has 1 or 5 or 10 names, he's one of the greatest keepers out there. Some names in this category are Courtois, Alisson, Lloris, Unai Simon, Musso, Ederson, De Gea etc.
The problem with goalkeepers is that you can be Gagliardini level and still be considered very good because your team always wins. Someone like Victor Valdes for example... So I get why people can get tricked by the Mendy hype due to that CL run and eventual win, but he's closer to Valdes than being a top GK.