*edit: sorry, misread that. My answer is not valid to your question.
original answer: because if you dont come up with these ints randomly they are sequential which can cause many unwanted situations where people can guess valid IDs and deduce things from that data. See https://en.wikipedia.org/wiki/German_tank_problem
Getting treatment or not is a decision. It is linked to pressure of suffering. You write that you did well and managed your life well. Awesome.
Other people feel extensive pressure of suffering and are not able to manage their lifes like you did. For those, professional treatment can be life saving.
original answer: because if you dont come up with these ints randomly they are sequential which can cause many unwanted situations where people can guess valid IDs and deduce things from that data. See https://en.wikipedia.org/wiki/German_tank_problem