Hiya!
The first thing to check is what you've set for itch.ios share in the settings: https://itch.io/user/settings/seller/revenue-sharing Since you can set it on a slider from 0% and above. Double check what it's at to see if that could be contributing to the large percentage being taken away.
Though I suspect the largest culprit probably lies within your tax information: https://itch.io/user/settings/seller/tax-information
Is the withholding rate 30%/40% by chance? I'm not from the US, so admittedly I'm not sure what the default rate would be for most from there, if anything at all, so others would have to chime in on that.
https://www.irs.gov/individuals/international-taxpayers/withholding-of-tax
https://itch.io/docs/creators/payments#tax-withholding
It's worth researching just to make sure they've got the correct rates set for you, and if you may need to update your tax information or not!