The legal routes are also often strenuous, far too long lived, overly strict, and obtuse.
If you barely speak the language, it's not easy. Unfortunately many countries have made immigration so hard, in an effort to combat it, that they've done the opposite - people immigrate illegally because it has a higher success rate than doing it legally.
If I apply for X, Y, and Z and I'm denied, I'm fucked. But if I just move... And then figure it out later... That might work better.
And that's how we got into this mess. A lot of this anti-immigration legislation actually increases the incentives for illegal immigration.