Because 8-10x handhelds are for finding game and helping to determine base line shooting decisions at short to medium ranges- not for detailed examinations and long range. Mid level glass absolutely has enough resolution, clarity, contrast and brightness for what a 8x or 10x big game bino is for. The time reduction between seeing an animal and getting a firing solution is greatly reduced with bino RF combos. So much so, that while a Swaro NL Pure 8 or 10x offers a phenomenal view- very pleasing to look at, and certainly can see more detail- the amount or percentage of times that the increased resolution will make the difference in killing an animal or not, pales in the percentage of time that game that could be killed due to the greatly reduced shot process that combined units give you.
15x binos and spotters are for gawking at animals- 8-1x binos are for killing animals.
Having said that, one can get great optics and great to good range-finding performance now.