Bcuz his father isn't, However he now have morgana the closest person to a real friend to him, but she also don't have a mother, so he thought for a while that this's a normal thing for every kid to grow up alone, until he met Leon, who does have a mother who care so much for him, but Leon won't just show as much because he's thoughtful and won't hurt Arthur, that's only Then that Arthur know, they (he and morgana) isn't normal, but so not every child have a father who is a king, and so he tried to convince himself it doesn't feel much different to have a mama or not. Until there's Merlin, his mama's boy, who is unlimitedly kind, care so much, love a lot and sacrifice unhesitatingly for him. and this's the closest feeling Arthur can experience about "unconditional love" that is what they say Mothers do.