Why? Various reasons; some (maybe a lot) come because they have relatives here that encourage them with the safety and opportunity (economic migrants), some because the local area is completely crime ridden and they flee the crime their local (often corrupt) government cannot control. {side note: yes much is drug traffic related; for years we have tried lots of ways to control the drug trade without any success -- is full legalization the answer? it will absolutely reduce the drug trade; and it will absolutely increase misery in the US}. Some must leave because the are targeted by their local governments. Note: the US views only the latter as valid asylum reasons. From this we can infer that they come from "very bad places".
So, let's take the next, logical, step in reasoning: how did those places become so terrible (something in the water, in the soil, probably not). It is the local culture ("how we do things around here") that allowed it. Just as our local culture rose up in indignation to stop child separation...it's not "how we do things around here". Just like #MeToo. Cultures do change. Podfish had the right idea to ask "can we change how they do things there so it won't be so bad?". And I ask "how do we keep the way they do things there from becoming how we do things here, so it doesn't become so bad here?".
Germans (modern, not third Reich) ask the same; And Italians, and now Swedes, and also British.
But Podfish also points out that we have a microcosm of the same situation in the US, with completely open borders and only a $158 Greyhound ticket keeping anyone in the US from coming to Sebastopol. So maybe, open borders would not be so bad, but it would be different.