The thing is: Terrorists are usually extremely stupid. It's kinda in the job description. Therefore a lot of them are caught before they can do much damage. Recent example from Germany: https://www.hessenschau.de/panorama/limburg-weilburg-17-jaehriger-soll-gewalttat-mit-waffen-und-sprengstoff-geplant-haben-v1,anklage-gewalttat-jugendliche-100.html

I'm not a big fan of the police (except when Sting is on the mic), but the authorities are doing a great job at catching them early.