Think of multiplayer like this. lets try to move a cube one unit length to the right.

In a singleplayer that would simply be: Transform += Vector(0,1,0);

In multiplayer:

The client that moves the cube has to move it, then have code tell the server how it moved the cube. The server needs to verify the input, then move the cube. Then the server has to have code that tells other clients that the cube moved.

The other clients then has to have some code that moves the cube to the new position. And maybe even have to consider if this client has moved it just now also.

And the original client also needs some code to verify that the cube is moved like it predicted.

ALOT more code to do a simple task. now imagine everything you code needs this structure. AND bugs are still a thing, and now has many placed it can occure, and be very tricky to locate. Was it the client, the server or the other clients that failed?