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Steam takes a cut from all purchases. This likely applies to microtransactions as well. Around 30% for smaller studios, a little less for large products. I assume the developers want to keep old players paying through their own website, with no steam cut at all.
Not that I play BDO due to pvp, but I'm curious as to why? Aren't characters tied to an account? Why not just log into your account when you boot up the game?