I get this same question about the profession I work in (special education). People go, "I'm good with kids, I live with a disability myself, the teachers out there are doing a bad job, and they complain there aren't enough special education teachers. So why won't they let me do it without all this teacher training and certification stuff?"
I think the answer in both cases is likely that there is stuff that goes on behind the scenes that most people have no idea about. Someone may know what works for them, and maybe even what works for most people. But will they know what to do when presented with a student or a patient who has, for whatever reason, extremely different needs? Will they know how to work with other professionals (if necessary) to determine and then implement a plan of action? Will they know how to deal with someone who has multiple issues which compound one another? With people (whether those they directly work with or relatives) who have not dealt with whatever changes need to be made?
Sure, someone can say, "I've been blind for 30 years and can teach braille better than X teacher." What they usually mean is they could teach braille to a student who is similar to themselves. But what happens when they are presented with a student who has a learning disability, a developmental delay, limited use of their hands (or all three!), or a student whose parents are not open to the possibility that their child could benefit from braille? Or a teenager who simply hates reading and refuses to put in any effort? Are they able to document their progress and communicate what they are teaching to other teachers, both those who are experienced with special education as well as general education teachers who may have no clue? Are they fully aware of all the responsibilities a classroom teacher faces and what is and is not realistic to ask them to take on (I certainly wasn't until I completed student teaching myself!). Are they able to take a student's progress, or lack thereof, and turn it into a plan of future action? Are they aware of and able to keep on top of research related to their area of teaching? Are they able to effectively document everything, including documentation which may be required by a particular district or government?
In most cases, when I've actually talked to people, the answer has been no. A teacher can't really be selective and go, "Well, I can teach X student braille because they are similar to me and they are enthusiastic ... but all the others, another teacher can handle them."
I think it's similar with a CDE. (And, I may be wrong, but I thought only a nursing credential was required? Not a full medical degree?) We see a CDE doing only what they have done while in contact with us. In reality, there is a TON more than goes on behind the scenes. They deal with a wide spectrum of patients, some similar to you but many very, very different. Not only different types of diabetes and different personalities and mindsets, but also compounding medical conditions which may have an impact on their diabetes control. Sure, maybe you in fact could do a fine job at being a CDE, but considering there are CDEs (and teachers) out there who don't do a great job (and there are also ones who are fantastic!), the certifying body has to make sure that there is some sort of minimal standard in place. If they open the test to anyone, I think the overall quality of the profession would decrease. It's not really possible to open the exam to you and not simultaneously open it to almost anyone who wants to take it. (You could say limit it to people who have had T1 for X years. But what about T2s? What about parents of kids with T1s? What about someone who is just interested and had done a lot of their own reading and studying?)
I will say that I do think it's GREAT when people with diabetes go into the medical profession. They have an insight and understanding of their patients that it's impossible for most medical professionals to have. I've found, as a special education teacher (currently working in the adult rehabilitation system) with a disability myself, students and their families open up to me in ways they don't to other professionals. So I think it's great! But I also fully understand the need for formal credentials. I'm working in a field right now that doesn't have any formal standards (assistive technology specialist) and while it's enabled me to enter the field and most AT specialists are highly qualified and fantastic, it's also enabled some others to enter who really have no clue what they are doing (yes, even though they themselves have a disabilities), which presents a huge disservice to the students they work with.
Anyway, just some (very long) thoughts from someone who has discussed such topics in a slightly different context! :)
