I would say because Rogues don't train that hard in terms of taking physical damage or being physically reliant. They operate from the shadows without hopes of you knowing they were there. They rely on Sneak Attacks, catching someone unaware. Yes, they can be played toe-to-toe, but if you're a Burglar or a Thief or an Assassin for example, you don't want to charge up in the field of battle and fight with a fully armored Knight. Though, game wise it is unavoidable.I'm personally not a fan of the idea of the monk getting d10 hit die. They're not meant to be frontline tanks. They're not meant to get hit often at all. Rogues don't get d10s either.
Where as Monks/Martial Artists for example would often train with limited to no gear to take on geared up opponents. Let's take the Taekkyon for example, an early Korean martial art used to take down armed Japanese opponents. Early Taekwondo with all of its kicks and splendor was used to take Warriors off of their horses. Thy came right into battle, empty handed, sometimes with farmer's tools as weapons, but trained to take on fully equipped soldiers.
Engage, Disarm, Neutralize.