Successive governments of many different nations have failed to secure their borders & enforce an effective deterrent to keep out the most disruptive and predatory of rotten apple overseas citizens.
Including international overseas crime groups (people traffickers, illegal drug/weapons/animal importers, terrorists).
The most obvious reasons tend to revolve around: complacency, corruption, incompetence, general lack of common sense, skinflint mentalities based upon false economy attitudes, widespread ideological sympathies with dangerously naive nonsense that aspires to a encourage a world with complete freedom of movement for the most entitled of humans. Presumably a world that can continue to function without any borders, citizenship restrictions and neighbourhoods with property owners that have no locked doors & windows.