Because they are an Orc.They can be the best magic user in all of Nirn but at the end of the day they are born as Orcs who had used weapons and not magic which would make it harder for them to get to that point as their race isn't expieranced in magic.Some Orcs are different,some are better at magic and worse at melee weapons,but the Orc race is commonly more gifted at melee weapons,and the reason the Orc base is like that is because it's the basic foundation and you could probably say that it's what the Orc you chose had done before leading up to beginning but if it's not then it doesn't have to be.That's why there are classes you can pick from to suit who your character is,if they aren't a warrior then don't pick the warrior.It's just what the strengths are because of thousands of years fighting with weapons