Why does my game quality looks substantially worse on my monitor than it does on my laptop screen?

I play Euro truck simulator on a Mac and I recently bought a monitor for it, but when I entered euro truck simulator, I was quick to notice that the quality of my game was very bad. My graphics was on medium so I took it up too high, but even that looked bad so I took it up to ultra and it still looks bad, also the render scale thing I forgot what it’s called is on 125% but it still looks grainy. Is there a setting to fix this problem? -also every other game looks alright it seems to only be Euro truck simulator.