It's so, incredibly crummy that you've had to deal with all of these incidents, and you have every right to be angry and frustrated that they happen.
I can't say why each of these guys chose to do this, since they'd likely each try to offer different justifications for it. One thing we do know is that, unfortunately, lots of men view women (and girls) as public property, meaning that they feel like they have a right to our space and bodies just because we're out and about. Too, there are still lots of people in this world who feel that their desire to do something to another persons body trumps that persons right to walk around unharassed.
Honestly, screaming or otherwise drawing attention to what's happening can actually work sometimes. As you saw with the most recent incident, the guy backed off when you showed you were willing to let other people know what was happening. Harassment thrives in silence and in places where women are taught to not make a scene or be impolite, not matter what. But if someone is touching you without your consent, "making a scene" makes sense (that's not to say that someone who doesn't yell is reacting the wrong way; freezing is one of the natural responses to these situations, plus sometimes it feels safer not confront the person).
The kind of punishment these guys could face if you reported them depends a lot on the exact nature of what they did, but it could be a fine rather than time in jail. Too, if you did report them, a bigger factor would be whether the people you reported it to took it seriously. I don't say this to discourage you, but so that you're aware that there are people who would dismiss these incidents as no big deal/