Normally this is done to keep things simple.
The company providing the reference doesn’t normally stand to gain anything by providing a detailed reference, it’s normally simpler and easier to just give the bare minimum.
It’s also risky to give a more detailed reference, if you say anything specific about the employee and they later suffer as a result, you’ll need to be able to prove everything that was said, if they sue.
So it’s generally safer to just say the least amount possible.