I've used the web on computers that were far, far less powerful than the Roku box. .
No, you haven't. The roku has a CPU straight out of the 80's. The perceived power is in it's GPU, which does video encoding nativity. It's hardware versus software where most computers use software and are therefore more flexible.
Think of it like this, a stove gets hot and can heat anything. A coffee maker gets warm but heats the coffee as it brews and over time on the pot. You can make heat coffee on the stove but you can't cook eggs on a coffee maker. The Roku is the coffee maker, it does a better job at what it was designed to do but has to stay specialized in the process.