That is according to ETSI requirements:
Wi-Fi channels cheatsheet
From channel 144 up you only have 13 or 14 dBm.
But for 6 GHz you have max 23 dBm (or if device is VLP - but this is not the case for an AP/router only 14 dBm), so besides issues with obstacles a 6 GHz should behave in theory like the low channels (36-48) of 5 GHz.