A murder of crows?

Today my husband informed me that a group of crows is called a murder. A murder of crows. It gives Alfred Hitchcock's The Birds a whole new dimension,  doesn't it?

A group of bears is called a shrewdness, a group of cows is called a kline (but a neat dozen is called a flink), a group of cobras is called a quiver (probably because that's what it would do to anyone who stumbled across it) but if it's a group of rattlesnakes it's called a rhumba. A group of ravens is called an unkindness while a group of turtledoves is a pitying and a group of rhinos is a crash. 

Some are descriptive enough to make some sense, like the rhinos, while others seem head-scratchingly random, such as rhumba-ing rattlesnakes.

Notice all the above definitions follow the same format: "A group of {insert animal name here} is called a {insert synonym for group here}". 

Why don't we just call them all "group"??