Because Buckets were already part of the crafting hierarchy and had a usage pattern - you could use 100 buckets on a 1 voxel of water and fill all 100 buckets. If this had remained then you would have been able to duplicate fluids by turning 1 voxel of liquid into 100, replacing 1 and gathering another 100. Then the fluids would have been infinite. This would have meant that fluids couldn’t act as resources. Water and lave are basically infinite, but we wanted the oil and resin to be more meaningful.
One change would have been that 1 voxel of liquid fills 1 bucket, but then the action of filling 100 buckets for crafting would have changed from 1 action to 100 actions - and players would have screamed nerf+grind. Additionally, if you have a stack of 100 empty buckets and attempt to pickup a water you then need spare inventory slot to contain the new full bucket.
Given that all blocks are already gathered by hammers / axes / shovels it makes sense that liquids are already gathered this way. Forging the attribute onto the tools is the most basic forge possible.
Additionally: the tools can already be forged - so gathering fluids can take advantage of 3x3 boons, etc.
This is because it’s a flexible way for adding new features into the game without always requiring us to add more tools, which in turn need to be crafting and carried by players.