Having checked the B818 a bit more, I don't think it's true 8x8, I think it's actually more like 2x 4x4, in that it supports 16streams from 4 aggregations, but maximum of 4 streams per aggregation - not 8 streams from 2 aggregations.
While the 3GPP Release 11 spec does define 8x8 MIMO, more than 4x4 antennas generally weren't available when the transition to (and therefore deployment of) 4G was taking place. Not many devices really had support for 4x4 it until the last few years, and use of rank 4 (all 4 streams of 4x4) on them is hard to get without perfect signal conditions so network providers didn't really have a reason to deploy it anything more than 2x2, at least until the last few years where UE support and data consumption is is rising.
Where 4x4 does help is on the cell edge as there is a bit more chance of receiving a stream compared to just 2x2.
The advent of 5G does bring benefits of higher than 4x4 antennas (8x8 and 32/64TRx MaMIMO) in that within the 5G specification it allows for beam forming to better target coverage to where it's required with a sub-set of an antennas' antennas. I don't believe the 4G spec had this, and so was relatively 'dumb'.