First, LTE-U with its fixed pattern is a quick n' dirty non standard proposal. It's not even acceptable everywhere, as some countries (Europe for example) do require LBT in the 5 GHz band, which LTE-U does not do. So I'll skip that part.
For the 3GPP standard LTE-LAA, a fundamental reason that it requires a primary channel in licensed bands is that the way LTE works does require an always-on, reserved channel. This is the way LTE do works in licensed bands: the channel is fully reserved for LTE, under full eNB control. Changing this fundamental assumption would be a major breakage, and much much more effort. The result would be very different. Whereas using LTE carrier aggregation, with a primary channel in a licensed band and using the unlicensed 5 GHz band for downlink only (UL has some added challenges that makes it unpractical) secondary channel(s) with LBT is not too difficult. It's an incremental changes, but does not require breaking a fundamental assumption. And in this case, LTE-LAA could perform like other technologies (to be checked, but possible on principle).
So even if it fits with the operator wishes, the assisted part is also the sane way to add 5 GHz support to LTE from a pure technological point of view.