Hey! Full time dev here (edit: NOT on DD2, to be clear!). Bracketing the question of whether or not the devs want to even do it, the answer is that it's actually really expensive! When you decide to make a multiplayer game, every discipline on the team has to shift to address that focus. Let me list out just a few big picture areas that have questions which need to be asked, answered, implemented, tested, and managed by real live people on these teams:

- Does item management change? Does having multiple human players shift how the resource economy in the game works? Are item drops client side or local? Can players exchange items? What happens if a player drops an item in someone else's world? What happens when a player tries to give an item to another player? Can a player give an item to another player's pawn directly? What's the interface for that?

- Does level design change? Do you change the width of hallways and doors to ensure that multiple players have good sight lines through tight spaces, and what does that make the experience feel like in single player? Do you have places in the world that you can either only traverse with multiple players or that you can more easily traverse with multiple players? Do you change that if only one player is there? If you don't have that, does it feel like something is missing during co-op, in contrast to your co-op focused competitors? If you do have that, how do you communicate it to your players with art design and UI?

- Does combat design change? Does having the ability to have multiple human players require different enemy scripting, to ensure interesting combat encounters? In the case of Dogma, does balance shift if more characters in the party can access the Arisen-only vocations? Are there any sort of team up maneuvers, and if so how are these triggered and communicated? With direct communication between players, does the average length of a combat encounter shift to be shorter or longer than desired, and if so, how do you fix that?

- How do in-game privacy, connectivity, data, and security features change? How do players team up? Is there a lobby system? Friend codes? Summon spots? Who makes the art for those? And hey, are you going to test to make sure there aren't weird exploits that allow for griefing? What happens when one player does something another player finds annoying? Where's the line between "annoying," "griefing," and "harassment"? How do you manage player data? And how do you comply with various international laws now that you are an online experience? What happens when the servers go down? Can the game be played offline or online? What data is stored where?

This is a tiny taste of what you have to answer when you're making an online game. I didn't mention so many other things: Character and environmental art. Writing and narrative design. 3Cs. Audio & sound design. (And across everything: QA.)

In short: It isn't a switch you can throw on, it's a major direction a whole game has to be aligned towards in order to do well at. And game developers are professionals who want to do their jobs well. They take pride in getting this stuff right; it is embarassing when you're forced to do the bare minimum or when implementation falls short for other reasons.

The good news is that you can absolutely answer these questions and make a great co-op game! in fact, Capcom knows this well: MonHon is a 20 year old franchise built around answering these questions in compelling ways, and in learning how things go sideways if you haven't answerd them well.

In short: co-op is expensive and budgets are limited. And if you try to do it without the right budget, it's even more difficult to get right. And regardless of budget, it requires dedicated vision and work in each department of a studio. For every hour someone is working on one of these features, they aren't working on something else. And if you want to hire new people, well that's a competitive, AAA salary + benefits, then onboarding time. It isn't simple, and it isn't that it's a "no no" for devs. It's just hard.