Based on my experience, it's because they are so hit and miss. If they work for you that's great, they do provide the best touch response and coverage.I want to buy this because I always see Whitestone Dome mentioned as the best screen protector for many phones and now its $15 on amazon but... why are there so many 1 star reviews?
However, I went through 5 applications on two different Pixel 3XLs (one at home, the others by professionals until they finally refunded me as well) and all of them had the spidering glue edges that made it hard to read the screen.
For $15 it's probably worth seeing if it works for you, but at the full price, I couldn't recommend them.