Because being hypnotised prophylactically can easily come across as creepy and controlling, even if it’s well-meaning.
Let’s give an example that is both well-meaning and at least a little overbearing: “Hey, let’s hypnotise our kids to really want to try hard at school.”
Freedom means the freedom to be unhappy, make mistakes, and differ from others, as well as the freedom to be happy, succeed, and conform.
If you want prophylactic hypnosis, maybe try self-hypnosis? However, from everything I have read and tried myself in that field, it’s still used reactively. You realise your thoughts and behaviour cause a problem (e.g. over indulging in some vice), and you try to hypnotise yourself to not. Imagining potential problems and trying to fix them ahead of time could be based on poor assumptions, and lead to you trying to change your thoughts and behaviours in maladaptive ways. Fixing a problem before diagnosing it doesn’t make much sense to me.