My speculation: there's no real market need. Devices that don't have enough space for the RJ45 are probably using WiFi anyway and have no burning need for a physical ethernet connection.
Since making the connectors smaller also makes them less robust, why take on that tradeoff in the absence of a clear benefit?
I understand that R-Pis and the like appear to be an exception, but they don't represent the amount of marketshare required to spur the sort of industry movement needed to develop Yet Another Standard.
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