I think the answer here is a bit subtle and hard to explain, because it contradicts a lot of common assumptions about addiction and drugs.
In short, many addictive substances create a chemical dependence that often has awful, even potentially fatal chemical withdrawal symptoms. Behavioral addictions don't cause this, which makes people assume they are entirely something different, and categorically less serious and damaging.
This is wrong- because those withdrawal symptoms, while they do make it harder to quit by making going cold turkey difficult and sometimes impossible, they are not the underlying reason why these drugs are being abused in the first place, nor the reason they destroy peoples lives. The reason is that they stimulate the reward system and/or allow one to escape negative emotions and trauma. Behavioral addictions also do that, and can just as easily ruin ones life, by completely overcoming someones mind and will, such that they no longer are able to live their life, and are unable to escape or quit with willpower, just as much so as with drugs that cause withdrawal. They can still completely ruin your life and drive you to suicide, etc.
Moreover, people also often emphasize that many addictive substances can directly cause serious health problems, or even death. This is also not central to their harmfulness, nor always the case. In fact, for a drug to have substantial abuse potential it must be relatively free from serious adverse health effects, at least in the short term, or else it would become impossible to abuse- the most damaging substances are the ones where people can take higher doses for longer with less adverse effects, because this more strongly emphasizes its ability to be used to strongly stimulate the reward system and escape negative emotions and trauma for longer periods of time - cementing the addiction-, without causing a new negative experience on its own. Methamphetamine for example is unique among stimulants in how benign it is- allowing people to take massive doses over really long periods of time, and not face immediate health issues. Counter-intuitively, this is actually what makes it have so much abuse potential and cause so much harm, compared to other stimulants which quickly make you sick or feel awful at high doses. From this perspective, you can see that the fact that behavioral addictions are also able to be repeated in "large doses" for long periods of time without immediate short term health consequences can make them have a high potential for harm in the long term.