ELI5: Why do old games have worse graphics than the ones that came after?

I mean, Minecraft came out in 2009 and has worse graphics than a lot of the games that came out in the last few years. I understand that they can change the graphics, but I guess my main question is: why are they worse? *I guess I’m referring to the worlds zbyxelatedness. What does that mean? What makes it worse? Why can’t they make them better?*