Can the braces act like a retainer?

I've had braces for three years, and I had to move for another country for year, I kept the braces on because I wasn't done. I couldn't continue my treatment with another doctor. So that I spent another whole year with braces on without any appointment with a doctor . That's 4 years. I've removed my braces right after that, and my doctor gave me the retainers and told me to wear it 24/7 for 6 months. Now I wonder , why can't the whole year with stationary braces help as a retainer ?

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