I think your screen size is too big. It usually happens when scaling down. I'm viewing it on a 13" laptop screen 1366x768 resolution.
The seams usually disappear when you up your screen resolution or screen size, but we cant guarantee everybody have such big screens or resolution.
Here's a screenshot.
The reason why we cut BG images up is because we have some border backgrounds with alot of empty spaces in the middle. So cutting them up will save a lot of wasted memory. Smaller tiles usually can save more memory for odd shaped borders.
Also if the image is 2048x1536, it will waste 2048x512 of memory. So, its become a common practice for us now to cut them into 512x512.