Why do my rose bush have such large thorns
Alameda County California roses
When it comes to roses and certain characteristics, it all comes down to variety and breeding. In most cases people who breeding roses are focused on the flower, may it be petal count, size, fragrance, size of plant or breeding the thorns out. With the rose you have you could have an old rose that the natural breeding provided dense large thorns to discourage rodent from eating or damaging the plant.
I have attached an information sheet that goes through the a few of the different varieties of roses and their characteristics. I hope this information will help you to understand your rose and the large thorns.