Why is the world quantum?

The world is quantum because the questions an observer can put to it do not all commute, so asking one and then another differs from the reverse order. That refusal is what ties observers together. If every question could be answered alongside every other, each observer's answers would be an independent row in a giant spreadsheet, and nothing one observer held would constrain what another holds. Questions that commute with everything collect in one corner, called the center, and that is where records live. That corner looks exactly like classical physics, a world of tables and pointer readings. A record layer like that cannot change smoothly and reversibly, so whatever flows in the world flows in the part that refuses to commute. With the questions fixed, the rule for odds has one possible form, the Born rule.

Also in Chapter 20: Why Does Nature Cheat, But Only By Exactly 2√2?.