This is much more about being pragmatic than anything else.

First for your argument to be true you would have to assume that people lack complete agency over their actions AND decide that they should never be punished for it ONLY helped. This would than go for every single crime no matter who no matter what.

I hope that we both agree that people do have agency and i hope we also agree that people have to pay their debts to society and that the threats of punishment is reflected in the desicions thst people make.

Further I hope we also agree that our solutions to problems in society should be effective.

When we help drug addicts we reduce the harm to society on all fronts and we can of course also make a moral argument that drug users hurt themselves the most and that addictions are mostly a complex beast that didn't come into beeing purely bc the addict wanted this to happen. We DO NOT ABSOLVE THEM of the crimes they commited bc of their addiction, they are still legally and socially punished for stealing or robbing or whatever else but we also reduce the need for these crimes by having these programms.

Rape on the other hand is a completely differnt crime, not only because of the harm they are doing to others but also because there is no actual dire situation where you need to do it and as far as i know, there is no "illness'' that makes you a serial rapist. Rape is about control, they want to hurt others, the hurt is the point and even if you want to classify this as an illness there is no real therapy for that. They COULD HAVE partaken in consensual rape play if they wanted to and when we have a rich guy they had all the money in the world to do so.

For an example, a pedophile should get all the help there is BUT as soon as they do harm to others they kinda lost that priviledge because they have proven they aren't safe for society. A pedophile does not NEED to rape a child they can live their whole lives without ever touching one, they might not like it but they absolutely can.

Not every crime is the same and not every bad person is mentally ill just because you think a normal person would not have done this.