MicroSD cards come in a variety of sizes, but the largest ones give us something to aim for. They're aspirational. No, I'm not going to spend $400 on one when it first comes out, but I know that someday, eventually, I too will be able to afford nice things. And sure enough, the price comes down. Then it drops again. And again. And again. Right now SanDisk's 200GB microSD has lowered to $135 on Amazon.
I hear you—that's still a lot of money. That's phone money. You can buy a Moto E and stick a 64GB card in it for that price. But on the other hand, sticking a 200GB microSD card in an Alcatel OneTouch Idol 3 is going to cost significantly less than a 128GB Nexus 6P. If you really want to treat your phone as a portable hard drive, the cost of this microSD card is steadily entering the range where buying one makes sense.