I understand WHY they do and it's simply because they can

It's not simply because they can. It's also because they have huge monthly costs associated with running the game. With non-MMO games, the game runs on your computer. With MMOs, the game runs on their computers. So the first cost they have is to maintain large numbers of computers in data centers.

Let's take World of Warcraft as an example. According to this site:

WoW’s infrastructure includes 13,250 server blades, 75,000 CPU cores, and 112.5 terabytes of blade RAM.

The Blizzard network is managed by a staff of 68 people.

So right away, they have to pay the salaries for a staff of 68 people. And they had to purchase over ten thousand computers, and replace/upgrade them as they wear out or break.

And all of these computers also necessitates paying for and maintaining very expensive Internet connections to accommodate the bandwidth necessary to send/receive every single action of every single player. The details about how much bandwidth they need, and what they pay for it, is not publicly available. But based on what I've seen of large business Internet infrastructure, they're likely paying tens of thousands of dollars (or more) per month just for their Internet connection.

And finally, games like WoW that rely on subscriptions tends to have longer release cycles on paid expansions, and instead have large amounts of contented created and added for free during an expansion. All of this content costs money to develop, and that is all paid for by subscriptions. Free-to-play MMOs tend to release content more slowly, and generally only add content in paid expansions, or have some sort of micro-transaction system to make money between expansions.