My pet theory is that a lot of AI's default "personality" stems from the rich executives who dream these products up. AI behaves exactly like the various sycophant advisors, admin assistants, servants, employees, and others who exist in these rich, powerful people's orbits.

Every human interaction they have in their day to day lives are with people who praise them and tell them they're absolutely right, and that what they just said was a great insight. So it's no surprise that the AI personalities they create behave exactly the same way.