"We don't really feel the need to add wireless charging. We're not going to add a feature just for the sake of it," said OnePlus officials when asked about the OnePlus 6. Apparently, having to buy a wireless charger separately is considered a barrier to entry, another strike against including the feature. And of course, the company points to the increased complexity (and price) of adding support for wireless charging. For many this is a non-issue. Dash Charge is just insanely fast and you can top off your phone for a few minutes in the morning and not worry about the battery for the rest...
Source: https://www.gsmarena.com/the_company_opens_up_about_why_the_oneplus_6_doesnt_have_wireless_charging-news-31192.php
