Is there a good reason for the modern trend of removing dividing lines?

Sometimes a website I use frequently will get a major visual overhaul. Recently (within the last few years) the trend seems to be removing dividing lines and reducing contrast. I personally find this trend quite annoying. I find it makes the interface much harder to comprehend/understand because there's nothing to guide my eyes, just random things floating in space.

Is there a legitimate reason for doing this, or are all these companies just abandoning good UX in favour of "cool factor"?

Case in point, Google Sheets. It was updated a few days ago with a new visual design, and while I'm sure I'll get used to it, it immediately felt harder to see what was going on. Here's a couple of screenshots demonstrating the difference. Is it just me or does the sidebar in the second screenshot not have any sense of being separate from the main spreadsheet?

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