We agree that this plant does look like a cactus. Some people even call it the African milk cactus. But the reason for this is that cacti generally do not have true leaves. Instead, cacti have spines, which are modified leaves. In contrast, Euphorbia trigona has leaves that grow between the spines that grow at the end of its stems. And while we're at it, it's not a tree either. It's just that these plants can grow rather tall and can be tree-like in structure.