Probably because these "fuckers" are UK Citizens.
The 7/7 attackers and the Manchester bomber were all "homegrown", this in an internal problem, not some external invasion.
It's a problem for multiple reasons
1) current foreign policy makes it easier to recruit in war torn countries, this gives them more people who can target at risk youth in western nations through social media and other channels.
2) the lack of on the ground policing is reducing the intelligence we are getting so while this person and the Manchester attacker were on lists the lack of actionable intelligence means they cannot be arrested. I do not want to live in a country that goes too far the wrong way and starts locking people up for thinking bad thoughts.
3) the drop in community support workers also means that targets for extremist propaganda are not having these ideas and beliefs challenged.
Reducing recruitment in war torn countries reduces the ability to spread propaganda, working with other nations can also help reduce the funding to these organisations. This will reduce the number of vulnerable people who are exposed to such extreme ideology.
Increasing policing and intelligence services means we can investigate those suspected threats in more detail and they will hopefully have more information to go on as well.
More social support workers will also help reduce the numbers of vulnerable people who are converted by the extremist views. That means there are fewer targets the police and intelligence services need to look at meaning there are more resources per suspect.
There is no way around it in my eyes but the cuts the conservative government have introduced are a contributing factor to these attacks. I also do not think you can trust this government to keep us safe when there have been 3 attacks in 3 months.
To be blunt, anybody who votes conservative is, in my opinion, partially responsible as they implicitly support policies that increase the likelihood of these attacks taking place.