Aesthetic design and usable design are very different things. If the sites you're talking about are made by people who care more about making things look nice i

Aesthetic design and usable design are very different things. If the sites you're talking about are made by people who care more about making things look nice instead of being usable, then yeah that would be the issue. WCAG2.1 has some great guidelines when it comes to color contrast in digital text and as long as those are adhered to, most people will have a near optimal experience consuming that text. And yes, too big of a contrast difference is not great either. Just try to throw bright yellow on top of a black background, it's gonna hurt your eyes pretty quick.