Originally posted by Honorable_D:

Ores/ingots are Valheim's primary means of progression, so the devs don't want mining to be a trivial matter. Filling up inventory and just portaling right back to base to unload and then portaling right back to the ore vein would be braindead easy.

It also ties nicely into the ship sailing mechanic by making sailing an important thing you do throughout the whole game.