The monitoring schedule after being cancer-free varies greatly from institution to institution and doctor to doctor. Most stop CT scans at year 5 and some doctors continue yearly scans. There are some guidelines that you can refer your physician to at: https://www.nccn.org/professionals/physician_gls/f_guidelines.asp#site and while they are not perfect, your MD can sign up for free and get a free copy. Your MD could very well be a testis cancer expert and have good reason for wanting to do the scans so the key is to just keep the conversation going with them.
Congratulations as you have surpassed 5 years cancer free, thats an amazing feat, I too agree with you that tumor markers should give you an answer and the tumor can be dead tissue, ct scans are just a way to make money to the hospitals and the oncologist, its bitter truth, if u r strong inside and living a healthy lifestyle and low stress in your life, nothing to bother is what I would say, healthy life is what is required for a cancer survivor.
I am still in my first year and under observation and will go for scan in a month or two but i strongly believe these medical institutions wants to make money as well and they dont care about the patient, the patient is just a commodity for them and their business. Period.