Q: Why can't my shipper give me an exact delivery date for my shipment?

A: Unlike airlines which have frequent take offs and landings and in case of a problem, substitute another aircraft once a ship is loaded it is only unloaded at the destination port.
This means any delay along the way will delay your vessel's arrival date. Delays include both the obvious and the less obvious. Obvious delays include mechanical breakdowns (a major engine problem can cause a long delay and your container will be stuck on board, enjoying drinks poolside until the engine is repaired) and bad weather (big enough waves have even been known to wash containers over board.)
Less obvious delays are caused by events such as strikes, port congestion and high winds at port. High winds can literally blow the containers around when they are being unloaded from the ship and are a relatively common delaying factor. (The boat will simply not unload during high winds.)
Port congestion often causes more than just a delay, shipping lines frequently add a congestion surcharge to compensate themselves for having to sit outside of the port and wait to off load their cargo. The Los Angeles port of Long Beach has been suffering from (expensive) congestion for quite some time. After the strike of the summer of 2005, Ashdod and Haifa also had serious congestion problems.
Shipping line issues listed above are part of the problem - once your shipment arrives in Israel, assuming your agent handles everything on a timely basis - our favorite government officials at customs enter the picture. I will try to not be too cynical when I point out that during the peak summer season, when we handle 70% of our annual business, most of the customs officials take their vacation. Makes sense, no? More work, so take a vacation, let someone else be stuck with it. Needless to say, files are cleared by customs more slowly in the summer than during the rest of the year.