Because there is a fee associated with every external payment transaction. Smaller the purchase, the larger portion the fee takes. This is the reason Valve set minimum price for software on Steam store to be 0,99$ full price and 0,49$ on sale a while back and humble set 1$ minimum price for their pay what you want bundles.

Let's say the fee per transaction is 0,1$+1,5% (that's really low end, fees vary depending on payment method used) of the transaction amount.

Single 5$ transaction would cost Valve 0,18$.
10 0,5$ transactions would cost Valve 1,1$ (0,11$ per transaction)

The amount paid is the same in both cases but in first case Valve would get 4,82$ after fees while in second case Valve would get only 3,9$. The fees my not sound like much on individual level but Steam probably handles million+ transactions a day on their store alone. On Steam market it's probably tens or hundreds of millions of transactions per day.

There are no transaction fees on Steam Wallet funds unless you put more money in from your external payment methods.

It should be obvious why Steam does not allow external payment methods in the market. In above example, any transaction under 0,11$ would cause Valve to lose money and real fees are likely to be higher than in my example.

Average Credit Card Processing Fees and Costs in 2025:
https://www.fool.com/money/research/average-credit-card-processing-fees-costs-america/

I could not find an article about debit card fees but some sites state it's 0,07$ fixed transaction fee per transaction, or at least was in 2023. No idea if there is variance in fees like credit cards have.

引用自 nicholaicaroll：

WHY CANT WE USE DEBIT CARDS OR CREDIT CARDS ON IN GAME PURCHASES??!?!!?

I'm trying to buy a cs2 crate and I try to select my DEBIT CARD option but there is no option for it, and I am NOT trying to waste 5 dollars on a 0.50$ purchase.

https://partner.steamgames.com/doc/features/microtransactions#2