They're definitely overcharging for more profit, but your margins won't be much lower if you handle payment yourself. Your 3% bank cut to receive money doesn't always work because often there's a minimum fee per transaction.
I know most iDeal (Dutch payment provider) transaction costs are 25 cents. It's a flat fee, so buying a €2000 TV will still leave you with 25 cents of fees, which is great for big stores, but when you use them do donate a single euro, the transaction fees are a whole quarter of the donation.
Tons of tiny transactions is a pretty terrible way to receive money. It's not "give up >15%" terrible, especially if your patrons tend to donate more, but Patreon needs to spread costs over all creators to make small content creators worth the effort.
Having a look at Donorbox, the same issue becomes clear: the cost for receiving payment in .NL is only 1.4%… and a €0,25 payment processor fee. These low processing fees are also only applicable to registered non profits in the NL which the author most definitely isn't. Even with the extra cheap rates and a non profit the fees to the lowest tier (±€3) add up to nearly 10% on a platform that's built around minimising costs for non profit organisations. There is the ability to use bank transfers for real cheap, but that's always a possibility anyway. One thing this site does seem to offer is the ability to offload all the site's cost onto the person who donates rather than subtract it afterwards, but that's just raising the donation price to compensate, not really a decrease in cost.