Well, Minecraft is developed by a team of about 70 people, and is owned by Microsoft, one of the biggest tech companies in the world. Fortnite is developed by Epic Games, which has over 1,000 employees. While Epic is much smaller than Microsoft, it's still a billion dollar company. Stardew Valley is developed by a team of two, plus the companies that handle the console and mobile ports.
Now, does that mean that you need to be a billion dollar company or have a huge team to create a game that has cross-platform support? Of course not. But it's also not fair to compare a very small company making a relatively small game, to massive companies making two of the biggest video games in the world. Minecraft and Fortnite will always have more features and be able to introduce new content far faster than a small studio could.
I would certainly like to see cross platform multiplayer added in a future update! I do hope this feature is added, but it's a tough one to implement and I'm not sure if it's something we'll ever see or not.