Because there is no reason to. And there is a very low chance he will be bred.

It can increase aggression:
http://www.saveourdogs.net/wp/wp-con...er-in-dogs.pdf

It increases unhealthy risks:
http://www.naiaonline.org/pdfs/LongT...uterInDogs.pdf

And here are a few more scientific journals and websites that has shown there are no benefits to spaying and neutering:

http://www.caninesports.com/uploads/...tions_2013.pdf
https://naturaldogtraining.com/blog/...ver-neutering/
Why I've Had A Change Of Heart About Spaying And Neutering | Retriever Life : Retriever Life
http://www.caninesports.com/uploads/...avma_study.pdf
Spaying and Neutering: New Warnings About Health Problems | Truth4Dogs
I refuse to neuter my dogs | PawshPal.com Blog