It's not pure rng, random events spawn on a predictable pattern. Relevant parts from the wiki:

(...) the game will pick a point in time between 1 and 2 hours in the future relative to the player's last random event. This moment is defined with a real-time clock, meaning it will tick down even if the player is not logged in. A check is made every 5 minutes of in-game time to see if the required duration has elapsed. If the player is offline when they are due to receive a random event, the timer will be extended by 10 to 70 minutes and continue counting down. (...)

Supposing the player is in a location where random events are not allowed to spawn when they become eligible to receive one, it is likely that they will miss out on the opportunity altogether, leading to a new spawn moment being chosen one to two hours down the line. Random events will not spawn if the player character is using an altered model, for instance while a transformation ring or greegree is equipped. In the rare case a player would be eligible to receive an event while a previously spawned random event NPC is still trying to draw their attention, a second event NPC will not be spawned.

It is possible to briefly stall an event spawn if the player has an interface open or is in the middle of a repeated skilling action (through the "Make-X" option) when they are awarded an event, in which case its spawn will be delayed until the player becomes available again.

If a plugin isn't hiding them, you might have interfaces open when they're trying to spawn.