I get your point but actually why can they lie and claim your buddy ratted you out?

The assumption, as I understand it is that a guilty person is more likely to confess if they believe the evidence against them is strong, but an innocent person will not believe the lie because they know they didn't commit the crime and generate the purported evidence.

I fear like many other old assumptions about criminal justice that isn't a close match to reality.

Tax payers don't care enough to force the congress critters to change the laws in <your home state>.

Instead, you get human beings who are shielded by a thin piece of paper who can summarily execute you, then say "whoops, my bad".

Police are nothing more than State-sponsored gang members.

I imagine a lot of it is tradition, AFAICT it was never really banned in the US (or Britain), although in the US it was codified in 1969. [0]

[0] https://en.wikipedia.org/wiki/Frazier_v._Cupp

Simple answer?

Because there is nothing prohibiting it.