It's the same reason we don't seek to improve crime rates by legalising theft. Sure, there'd be fewer people labelled as "criminal", but the original problem would remain (and in all probability would become worse).
Obviously, it is the reason that matters and the reason is not benign.
People want other people to like people in certain way they and then castrate Alan Turing for illegal love. People want to have slaves then they have illegally free slaves problem.
Why pretend that this is about upholding the law?
https://en.wikipedia.org/wiki/Dublin_Regulation
Basically economic migrants - predominately young men from the middle east - disguising themselves as Refugees and taking social supports away from the families fleeing warzones.
In other words, this is actually welfare fraud that happens to have a travel component.
As a philosophy of law point, aren't laws passed to make things illegal if they aren't wanted? Rather than legal?
"it's not that I don't like them - I am not like that, its just that I don't want illegal immigration"