Why is it so hard to predict arrival times?

[Peter47]

I've spent a long haul flight at the ETA on the moving map and its amazing how it varies from hour to hour. Some ideas for this:

Changes in altitude, head/tail winds, etc.
Flying off course (I've been in some quite long diversions around thunder storms and you might not get the requested route
Being stuck below a slow flying aircraft (a pilot told me how they couldn't change altitude to pass a slow flying 767 (Mach 0.78?) - cost a lot of time
Being stuck in a stack
Change in landing runway direction - could easily cost 10 minutes, more if it leads to additional holding
Getting a straight in - I saw the eta into HKG magically reduce by 15 minutes at a stroke once
Changing the cruising speed - aircraft often fly below max speed to save fuel. I was once delayed 2 hrs in HKG when the plane went tech (day flight to London). Guess what, the flight time went down by 20 minutes - it may have saved breaking the night jet ban on the return flight.

Of course some of these are less likely to apply to a SH flight like MUC - BUD where the arrival time should be reasonably well known once the plane takes off.