I believe the reason is, at the frequencies these CPUs are talking to RAM, the reflection coefficient[1] starts playing a big role. This means any change in impedance in the wire cause reflections of the signal.
This is also the reasoning why you can't just have a dumb female to female HDMI coupling and expect video to work. All of such devices are active and read the stream on the input and relay them on the output.
See also RF insertion loss and how that's dealt with, PCIe retimers, etc.
Above certain frequencies, you start running into major issues with signal integrity, and fixing them is very difficult without any extra circuitry.