It’s not that uncommon in certain regiments in the British Army (especially infantry). If somebody wants promotion taking eight years to get a stripe would be very slow, but they don’t force somebody who is happy being a Private to seek promotion if they are doing the job where they need to be doing it.
A person can be an excellent soldier, without necessarily having the inclination or ability to lead others.
You are kind of a rare bear. I only ever knew two, or maybe three, Spec 6’s. I made Spec 5 in nineteen months. Then I was put in an E-6 job but didn’t have sufficient time in grade for a promotion so they made me an E-5 hard stripe.