I have a pretty decent computer, a Macbook Pro from 2008. But it isn’t powerful enough to drive many modern games at the native resolution of my external display (1900×1200).
I could reduce the resolution, but the problem with reducing resolutions on an LCD is that anything other than the native resolution looks like garbage. A full screen game at 1280×800 resolution has to be rijiggered for a 1900×1200 display and it ends up looking awful.
But then there’s window mode: an option for games to play in a window instead of fullscreen. This way you can at least get the game in a playable resolution, but you still have to play it in a window.