Our world = Evil people are those who like evil actions and/or are addicted to them. They can change at any time, but they don't because they like evil actions. There is no nature or dark energy making them crave evil. They chose to enjoy it, and they continue to enjoy it.
Fantasy worlds = Evil is represented by actual forces such as evil spirits, the Shadow Weave, gods and evil creatures, like dragons. Evil magic can corrupt and literally change people so they have no choice but to become evil. This was literally the premise of BG1 and 2. You were fighting the powerful corruption heritage within you.
D&D is built upon alignment systems. As you aquire items, you atune yourself to them, but only if you share the alignment. A good person carrying a corrupt artifact may find themselves being corrupted.
So, in fantasy, an entire race can literally be evil because the race as a whole is attuned to evil.
You are certainly welcome to your opinion of what makes good fantasy.
You are certainly welcome to misrepresent the posts of others.