Why isn't LAN/RJ45 an ubiquitous input into DACs?

From the surface, which is where I sit, it seems like a good option as nearly all computers have RJ45 and there is practically no limitation in terms of cable length, yet it is unusual to find a DAC with such an input. Why is this so?
Because it is a major undertaking. One must design the GUI for organizing a playing tunes and the network interface S/W for the computer and then the network interface for the DAC. Look at SB Touch and Sonos. They are completely self-sufficient and proprietary. If these were open systems, the lots of DACs might have these interfaces, but they are not.

This is why Apple or Microsoft need to offer a ubiquitous open solution.

Microsoft has taken the first step by offering "Play-to" in Win 8. It uses the DLNA standard. If one has the software and hardware expertise, one can design a "DMR" or digital media receiver. There are a couple of mid-fi companies already offering this in receivers. However Play-to does not allow for multicast or different streams to different devices, like Sonos or Touch. It is playing catch-up.

Steve N.
Empirical Audio