It sounds like you could be making the wrong assumption.
The credit card companies, PayPal, and Stripe charge a fee for someone to buy your game. That fee is carried over to your account. Once every three years you must complete a tax certificate which costs $3 (last time I checked).
See: https://itch.io/docs/creators/payments
If it was tax, your tax certificate would highlight that. If your country has a tax treaty with the US there won't be any tax withholding, only the fees. The fees can still constitute a large chunk of your earnings if you have a lot of small transactions.
In contrast, other companies absorb the fees in their 30% commissions. But because Itch.io doesn't enforce a commission, they can't pay for the fees themselves.