1) There are plenty of standards in the pallet industry, especially in the EU. (http://www.palettes-cp.com/welcome-en.html) The US has the 48x40 GMA which for many years was used almost exclusive in the grocery chain. These standards were born of similar or exactly the same needs from different customers. Things like 2x2 barrels, poly supersacks, square and octagonal boxes, etc. All those things are standardized and so the pallet that goes underneath them is largely the same.
2) Customization is a factor of efficiency. Why would someone shipping a product want to buy a standard size pallet that doesn't fit or hold the weight requirement for his use case? Instead, he consults with either internal packaging specialists (in big companies) or a manufacturer directly to design a pallet to fit his load. By doing this he actually knows the limitations of the packaging (stacked x high, racked, weight distribution, unstacker and rollerbed compatibility, etc) and also has some general idea that the product he is receiving is correctly priced for his requirements. Many times those requirements are customer driven, meaning my customer's customer.
Source: I make pallets, lots of them.