Why can't we just permanently change the default start like we can in FSX?...At least you can permanently change the default start in FSX.
You easily can.
Open up P3d. Go to MENU > Flights > Create Scenario.
Change your location, aircraft, time, weather, etc to whatever you want to be the default.
Click on "Save" in the lower left corner.
The following screen will appear. You have to click on the "default" button before you save it. Once saved it will then become your default flight.