Because some sellers don’t respect sales embargo and/or some clients have some privileges.

People in the USA have been picking them up for weeks from Costco. I got mine ~2 weeks ago.

I saw this set on the shelf in a big toy shop (Smyths) here in the UK a few days ago, didn't realise it was hot off the press.

One can definitely go pick one up off the shelf here in the UK as of at least Wednesday (the day I saw them).

beyond the other explanations that some have given, lego direct order releases ship from more than one warehouse, even in the US - my bricklink western train set shipped from Vegas, and others I had chatted with either got them 3-4 days earlier than me or 3-4 days later, depending on how close they were to the warehouse and the order of the orders going out.

just logistics, or luck as some of us call it?

Seems pretty lame to literally pre-order something and get it significantly later than others who haven't pre-ordered