There's a big reason I like to avoid going gift-card. Especially for osu!.
This introduces middlemen into the economic structure. You would have to pay for the card itself, the shipping costs to move the cards into stores, taxes atop of that and then the shop needs at least some money for services, so they're going to up the price a bit. If you ever saw the activation fee on a prepaid card, that's the kind of stuff you're paying for, other than the taxes. It would introduce a lot more expenses and time requirements.
Direct payments allow you to pay up front directly with zero middlemen.
It depends, do you want to pay an extra few dollars to drive to a store, grab a card, and get home?
Not to be sarcastic, but a lot of companies do this. It's getting obsoleted by online transactions very quickly. If you feel unsafe, you can use a payment management service like PayPal, and almost any website recognizes it. And even then, if you encounter anything, you can contact your issuing bank.
I'm not going to speak up for peppy, but for these reasons above is probably why there isn't an osu! Gift card and likely never will be.