The problem is possibility space (or "probability space" depending on which school you subscribe to).

With each new point of interaction, your possibility space in a game jumps dramatically and takes another step towards the infinite.

To illustrate this in a simpler way, say we're playing a game with D6 dice. Giving you a single role of one presents you 6 possible outcomes. This is a fairly limited game. So we give you a second die and let you re-roll one time, presenting you with with 36 possible outcomes. That's a lot for a game. Now if I add just one more die, your possibility space becomes 216.

The math of this is not profound by any stretch. But looking at this, I want you to ask how many dice you're rolling with each single step in an open world game? Because each action you inhabit, however small, is a possibility space.

Figure that at any moment your character may have: * A weapon with $customizations * Armor with $customizations * A questlog of $value * A follower of $value * Objects of $types, $number * Attributable sound, lighting, and particle effects

Figure that the world existing within draw distance will have * Archietecture * Weather * Light elements * NPCs taking $action alone * NPCs taking $actions together * NPCs taking $actions with the player

How many dice is that? How many times are you rolling? Again, this an over simplifaction. So lets look at Fallout 4 as an example.

What happens if I collect every tricycle in the game? Now what happens if I put them all one this in the corner of the map? What happens if I'm wearing power armor doing it? Now what happens if Nick walks into that room? Or an NPC? What if I have to shoot a rad roach in the room filled with tricycles? What if I take just one out during all this?

If this looks ridiculous, it's because it is. This outcome exists in the wider probability space of a game like Fallout 4. It's partly why its so unstable; the possibility space never limits to prevent you from doing something this needless.

Worse yet, you absolutely cannot test for all possibilities. We complain about how games like Fallout 4 are broken, but you cannot possibly test for every potential user interaction. You can just hope that enough are Kolmogorov conditional and dependent that you can catch the majority through like testing (in other words, if I put all the tricycles in a room of $size and the game doesn't crash, it's probably fine.)

But you never will catch them all. Not be a long shot. There exist more possible hands in a deck of cards than there are atoms in the universe.

This is why open world games like Arkham City and Knight are starting to incorporate world destruction as a means of cutting off prior interaction points. It's a smart idea, because a major time suck is testing for player who backtrack with high-gear to take on early content.