Bank transfer is the standard option because everyone has a bank account and everyone can do it, also, there's generally zero fees attached. There may be more convenient options, but it basically always is there as a fallback. As a company you have an account, anyway, only thing you need to do is have your payment system look through incoming transactions and scan the "intended use" field for a transaction id or account number or such you told people to put in there.

I have a bog-standard bank account and yes of course it comes with a debit card that doesn't mean that it works with the US-centric "enter card number and expiry date" system, though. Way too insecure anyway.

Steam manages to use Giropay, I can understand if a US company doesn't want to deal with that kind of solution^1^ but accepting SEPA transfers is dead-simple, dirt cheap, and covers 100% of the EU (and more) market.

^1^ The German banking sector, alas, in in the habit of pioneering stuff and then be incompatible with what big financial players elsewhere come up with. Other times the rest of the world simply doesn't care, e.g. when it comes to HBCI/FinTS.