Most people don't care or don't know about it. They just want better performance and they think that Crossfire/SLI is the way to go. Some also don't want to spend too much and since they already have 1 card, it's cheaper buying another one since most probably will cost less as time goes by.
There are other problems not mentioned in my previous post. You need a proper motherboard that does support multi cards, driver conflicts etc.
Another guy in this thread:
Hello! I have recently bought a new CPU; The Ryzen 7 2700x paired with 16 GB of DDR4-3200 and an Gigabyte Aorus X470 motherboard. I have an Asus GTX 1070 8GB GDDR5. I play at 1440p @ 165 hz. Now my graphics card is struggling when going on high/ultra in most newer games (like Apex Legends)...
forums.tomshardware.com
wanted to know about 1080ti. Read that segment.