[michaelt]

Because when you're sending a signal down a wire or through the air, fundamentally the communication medium only provides "Send it, maybe it arrives"

At any time, the receiver could lose power. Or a burst of interference could disrupt the radio link. Or a backhoe could slice through the cable. Or many other things.

IP merely reflects this physical reality.

[kmeisthax]

Ok, but why does TCP exist, then? If we could make streams reliable in the late 1970s why didn't we apply that to datagrams as well?