It makes sense when you think about it, upstream is typically in the like 5-40mhz range, where downstream/tv is in the 40mhz-1ghz range. The splitting and routing is done at the analog level, similar to how a low-pass filter routes low frequencies to a subwoofer in a high-end audio setup.

You can’t just have a hardware low pass filter start filtering upstream traffic above what the equipment is designed for, and with frequencies that low there just isn’t the bandwidth for the throughput people want.