I don't think there are as many proper incels as you think, tbh. There are a lot of incel aligned men online, but ones that are heavily invested in incel shit? I don't think there are so many millions of them. Considering that, yeah, their ideology has created a comparatively large amount of monsters. But mass shooters aren't the only incels who are committing horrendous crimes. They're just the ones who receive the most attention. I would be willing to bet that there's a decent amount of violence within their personal lives, or even murders, and either because these offenses haven't been explicitly attributed to their inceldom, or because they haven't killed as many people, it isn't commonly reported. I can almost guarantee that a lot of these men are violent. And plenty of "incels" aren't even incels like they claim. Plenty of incels have had girlfriends before or have women interested in them at some point, they just fuck up their relationships because they can't extract everything they want out of women and abuse them at the same time, and women put up with it as often.
>they just seek companionship, but since they cannot get it, they vent their frustrations online
100% untrue. They are completely capable of getting into relationships but they grow to hate women because women now push back on emotional dysfunction, abuse, and entitlement. Their gripe is their perceived loss of power in "Western" relationships because of the "masculinization of Western women", kek. The "rejected sadboi" narrative is apologia that's completely detached from reality. Yes, they say they just want relationships, but the reality is that they want women way out of their league, and they also want the ability to mistreat and dehumanize them. These things are no longer publicly and commonly acceptable in relationships and incels are pissed about it. They also use women as a convenient scapegoat for why quality jobs are dwindling, etc. These are the same men that will scapegoat minorities and make villains out of anyone to escape accountability, or to just avoid holding the actually powerful parties accountable. They aren't resentful because they "just want a relationship". That's the overly charitable view they want people to have of them. People who are sentimental and lonely don't encourage murder and hatred because they're just "so sad uwu". That shit makes no sense and it certainly doesn't match the things that they say and do in their communities, sharing photos of dead and raped women for laughs, etc. They are emotionally shallow bullies that are extremely angry because it's getting very slightly more difficult for men to abuse and mistreat with impunity. There's nothing else to it.