A source within the Wireless Power Consortium has told the Japanese publication Macotakara that since Apple does not support the Extended Power Profile that’s in version 1.2 of the Qi standard, the charging rate will perhaps only top out at 7.5W instead of the maximum potential 15W.
The Made for iPhone program allows third-party accessories makers to sell Apple Watch wireless chargers at the 7.5W maximum, so it could be that the case that certain wide surface chargers made under that license should be able to charge the iPhone 8, iPhone 7s, iPhone 7s Plus as well as the Apple Watch simultaneously.
A better question to ask? Why not change the standard and support faster charging? Apple may favor a more conservative when it comes to a potential USB-C power adapter that will connect to iPhones through the Lightning port — 10 watts.