To make the player work for rewards and not be simple overkill game. Makes you build up in each tier before moving on to the next. Each boss unlock something new for better gear to craft. So blocking on pick axe in beginning stop players from getting too powerful early in the game. Even skipping other bosses.
Deer boss-basic gear, defeating the boss allow to mine copper/tin for bronze
Black forest boss-bronze/troll/copper items, defeating the boss allow mine iron
Swamp boss-iron/troll/root items, defeating the allows silver finding (silver is very powerful in swamp).
Dragon boss (mountain)-silver/fernis items, defeating the boss allow to make blacken metal bars.
Plains boss not sure what you get. Padded gear/fenris/blacken metal items.

If you did not kill each boss for drop items, you can become very powerful and then lower boss become a cake walk. Just like making haul ore around in a ship or even eggs. No short cuts. Makes the game more challenging then simple x for items. Working for rewards is better than having everything given to you.