My opinion is that is boils down to a few things, in levels of decreasing obviousness:
(1) "Modern" design is "standard" design. There's no texture to contend with, just colors to swap in and out and placeholders for stock photography. All documents are styled as a set of disconnected templates within a frame with no consideration for how users find the information they want. Navigation bar, hero image, "content" (with sidebar if you're feeling spicy), fat footer, and 14MB of tracking scripts. That's all of modern design, and yes, it's as easy as it sounds. Your "brand new" website is nearly entirely recycled and hacked-together pieces of many, many sites like it. And it's "good enough" to ignore how users navigate the site because they're all coming in from SERPs anyway.
(2) Design documents are dead. You, as the developer, are expected to piece together said frame with a handful of .xd files and loose notation stuffed in a Google doc somewhere on top of a pre-built framework. The final product is schizophrenic, barely held together by whatever plugin was decided to be installed today without testing or consideration for the site as a whole. And as things are haphazardly tacked onto it over time it only gets worse for the end user.
(3) Consideration for reasonable hardware and backwards compatibility is extinct. If you are reading this on a 15-inch screen, you don't exist to the designer. He is using a screen no smaller than 22 inches with a high DPI setting on a fast network connection. When he loads another 3.8MB JavaScript library from his SSD and shoves it into the prototype, your struggling 4G connection or otherwise perfectly good laptop from two years ago is of no concern. When the cookie consent popup appears and inevitably covers the content you came to see, it doesn't matter because 70px of padding on every element is chump change on the designer's ultra-wide monitor. When you can't log in with your iPhone XIII because the the iPhone XV came out fourteen picoseconds ago, you don't matter because the requirements were only to support the latest browser version.