First of all it is not just longsword proficiency, it is longsword, battleaxe or warhammer profiency as all of them can do d10 damage.
Second most races CAN get one of these at level 1, just like they CAN put their +1 or +2 bonus into dexterity. Whether they do or not is a completely different discussion and if you want to compare without considering what "can" be done then you should assume a flat 10 and no ability bonuses at all.
Finally no character, of any class, can effectively wield a greatsword, glaive or maul without a racial trait too. Even fighters and barbarians can not use a great axe or a great sword without disadvantage unless they have a racial trait that makes them medium or large size. So like a Monk longsword, being able to wield a greatsword is also something a barbarian can only get from a race! If you are not going to assume the best weapon for a Monk because he needs a racial ability to get it at 1st level then don't give the fighter a weapon that many races can't use either.