Ah, the curse of not being in a bigger club strikes again.
Don't think that's valid overall. That's a new wave situation. There's definitely a tendency now, but it wasn't the case always.
Don't think anyone claims that Zico is a fraud because he barely played for Udinese and some Japanese team apart from Flamengo. He's still considered a top 5-10 Brazilian ever.
As with goalkeepers, the issue there is that eventually a great goalkeeper will end up at a top team.
And if he's not that great, he won't stick around. First example in mind is Rustu Recber post WC2022 at Barcelona. And yeah, GK spot are very limited and exclusive. And this is why.
There are also cases like Valdes where the team can just get by without having to spend on a keeper for various reasons, so during their best years, some top GK could have made a name for himself at Barcelona or somewhere else (domino effect), but Valdes was holding up a roster spot until they lost Mascherano and Puyol and realized that they needed someone who could be above average.
Thomas Ravelli and Michel Preud'Homme. Rinat Dasayev. All these are considered some of the best GKs of their generation (80s, 90s)
They didn't play for top teams as you would perceive them today.
If you think of the top 15 goalkeepers now, you would say that at least 13 of them play for top 15-20 teams, no?
Eventually the other two will join top 15 sides, unless they're Brazilian who don't want to cross the Atlantic or they play for their favorite team that just doesn't rise above a certain level.
The other thing is that any GK can look good at a given day. Anyone can have a great international tournament.
You're looking for consistency though. Lots of people fall under the impression that a GK is great because their limited exposure to them allows them to make that assumption.
There are very slim margins.
Edouard Mendy for example made everyone think he was a top GK. He wasn't. He just was part of a well oiled machine at Chelsea and made some good saves in the process.
Ranking players is an absurd Americanism, but arguably Mendy was a top 50 GK at the time, but his exposure in CL and the PL made people think that he was top 10 or better at the time.
So while you're right that big team "curse" may be a think when it comes to legacy and public awareness, the same can be applied to Handanovic.
If the CL was exposed to him during 2011-2017, they'd be calling him one of the best ever, not just one of the best of the tournament.