Why batteries need to be custom and smart nowadays?
Is it because of (some ideas):
Safety, because of possible shorts by wrong polarity of unprotected cells;

Sure, that's the main reason. A Li-Ion cell isn't consumer-friendly, as it may explode into your face when shorted or put into a pack with wrong polarity. It needs a protection against those cases.

Avoid unpredictable results of the product by using low quality batteries*;
Dependancy, you need to buy original (overpriced) customized battery pack;
Sales, control failure and replacement (like ink cardridges, black gold);
Product life cycle, product become obsolete when no battery available

This doesn't even work for ink cartridges.