It's simple math. A 32" 4K monitor is about 130 PPI. Retina displays (where you could reasonably say the pixels are not noticeable, and the text is sharp enough to not strain the eyes) start at 210 PPI.
Subjectively, the other problem with 32" 4K (a very popular and affordable size now) is that the optimal scaling is a fractional multiple of the underlying resolution (on MacOS - bizarrely I think Windows and Linux both know how to do this better than MacOS). Which again causes blur and a small performance hit.
I myself still use an old 43" 4K monitor as my main one, but I know it's not great for my eyes and I'd like to upgrade. My ideal would be a 40" or 42" 8K. A 6K at that size would not be enough.
I am very excited about this 32" 6K Asus ProArt that came out earlier this year: https://www.asus.com/displays-desktops/monitors/proart/proar... - it finally gets Retina-grade resolution at a more reasonable price point. I will probably switch to two of these side-by-side once I can get them below $1K.