That’s a bit more effort 
The default OS A10 is via Google, Fairphone have to go through around half millions checks with Google before they licence the OS. Apparently if you then unlock the bootloader Fairphone have to pay Google for each instance.
Here is an example for the recent A9 for the FP2
“To get Google certification for Android 9 for Fairphone 2 just as we hit five years of support for the smartphone is a huge achievement for Fairphone,” says CEO of Fairphone Eva Gouwens. “In order to get certification, we had to pass approximately 477,000 Google tests.”