decentralization is not some magical binary property. Real world systems can be more, or less, decentralized.
To start with, yes banking giants exists but everyone finds this undesirable to a certain extent and regulators would object if suddenly eg elon musk decided to buy out goldman and chase.
banks are highly regulated and enjoy completely different economies than email. Banks of the same nation are often legally required to be interoperable with others. Yes, there are still big players and you can eg get sanctioned by the US - which is why BRIC countries are working on alternative banking networks.
with banks, most jurisdictions would disallow a merger of two large banks specifically from decentralization considerations - guess what would happen if two email providers merge? exactly nothing.
With email, if gmail decide to ban you, you are unable to communicate with 90% of people with no recourse. A support engineer can decide to ban you on a whim and besides crying on social media, you can’t do much.
With banks, you need a large and powerful nation to tell other banks not to play with you, and you can contest this decision legally.
Most importantly, with banking money is the product and every transaction brings revenue, so there’s a strong incentive to cooperate. With email - and AP - cooperation incurs a cost on all parties instead and benefits are distributed and vague in monetary terms.
Finally, banks are required to deal with you as a customer - provided you meet certain requirements. An email provider can ban you for no reason and your only recourse is arbitration.
Compare to bitcoin. Will you still be calling it decentralized when one pool controls 90% of compute (i don’t think this can really happen tbh as there are disincentives to go 51% without abusing the network)?
Don’t get me wrong AP sure feels in the right direction compared to eg FB. Just on the principle of being able to run it completely in your own. However it would be nice if whatever new alternative that grew popular treated users, rather than providers, democratically.