Maybe because that B.S "low power consumption" song that AMD has been singing is a crock for those who want performance. These cards are already developing a reputation for throttling with the stock crap blower coolers (under STOCK conditions, let alone OCing), so I suspect even with their supposedly "more efficient" GPUs, these things NEED serious cooling to tame them.
These new 5700s seem to be just like the Radeon VII was, terrible with stock cooling and needing water to really get the performance numbers up. It will be interesting to see how AIB's air cooling solutions perform though.