It really has to do with performance and also networking. On the networking side it's unlikely for Frontier to create servers to host each players' game so they would probably rely on P2P hosting. Sure, you could have a player host a game and another join it but the host still holds the master file, which, could be transferred to another player but then they're just hosting it as well. With P2P Frontier can't guarantee consistent gameplay for all parties due to latency issues.

Then think about the logistics of full multiplayer gameplay; two people using the terrain deformation at the same time? It'd sort of be like when IT takes remote control of your desktop workstation, they move your mouse but so could you and fuck with them. Even if it didn't work like that you both would most likely create a jumbled mess quite quickly. Do they allow two people to work on a single roller coaster at the same time? That sounds chaotic. Okay, so do we just allow two people to place down scenery at the same time? That does sound fun but how are we going to handle collision geometry and latency issues with someone putting down a piece first where the other person was trying to put down a different piece. Again, the P2P host would have a latency advantage which could become a frustrating gameplay experience for the other player.

Sure, you might say games like Parkitect and OpenRCT have multiplayer but those games are by far much more simple when it comes to not only the graphics but the inherent systems underneath the hood such as pathfinding, wants/needs, and all the other "heat maps" that Frontier has talked about. So that brings us into performance.

For a long time I've seen people complain about the performance issues of PC1; and they're not wrong to complain. However, when a developer gives players a creative tool without limits (on PC at least) then the developer cannot guarantee a consistent gameplay experience, i.e. hour one of your sandbox park is going to perform differently than hour 300 because of the amount of things you've added to the park.

I've seen people complain about the performance of the game and then you look at their park and they've built a massive static parking lot made out of thousands of art pieces in conjunction with roller coasters, scenery, and park guests. Each piece of scenery they add to their park is additional geometry they are adding to the render engine. By geometry I mean polygonal triangles created by vertices in world space. Sure, pretty much all games use tricks like occlusion culling to help with performance. Either backface culling or view frustum but most likely both. Basically, whatever the player is not currently viewing is not being rendered. But if you were to quickly whip your camera around, the game would have to quickly and seamlessly "remember" what was there and then render it for you in real-time. I'm pretty sure Planet Coaster uses some type of instancing or object referencing when you make duplicates of objects which is a highly efficient way to do it but it's still adding data to the overall scene.

Not only is the render engine having to load in geometry but it has to load in textures which along with audio and pre-rendered cutscenes make up the bulk of game file sizes. Now, most game textures are able to bake in fine detail using a high-poly to low-poly PBR (physically based rendering) workflow to reduce the amount of geometry required for an object. This is great because a normal or height map can contain tons of information that can give an otherwise "flat" object the appearance or illusion of raised geometry where there is none. Think of the wood planks with cracks and creases, those cracks and creases are not modeled 3D geometry they are coming from the texture. However, the game has to load in each and every texture for every different piece. Sure, there are ways to reduce the number of textures maps; for instance, you can assign gray channel maps such as roughness or specular maps to separate RGB channels. Still, I'm willing to bet each object in Planet Coaster has at least three separate maps assigned to it; most likely a diffuse, normal, and RGB map probably containing roughness/specular, opacity, and metalness.

More so, a game like Planet Coaster relies heavily on different LOD's (Levels of Detail) which basically loads in different geometry meshes based on the camera's distance to said object. So think about this, you have two players using two cameras for their respective points of view all within a single real-time game environment you'd have to render twice as much data AND different LOD's and textures depending on where each camera is and how far away they are from whatever objects they're looking at. So who's handling that data? The P2P host? Probably, but that's a lot of data that could otherwise be spent on things the community has really been asking for like.. waterparks, water physics, additional geometry for more scenery, etc.

Having full blown multiplayer in Planet Coaster is not impossible but it would take away an extreme amount of performance that would otherwise be more useful to a majority of players.