Terrorists fill a need. Modern conventional war is increasingly impractical. It is too destructive. It is too expensive. World and sometimes domestic opinion imposes constraints.
Terrorists offer a possible alternative to open armed conflict.
For some nations unable to mount a conventional military challenge, terrorism is an 'equalizer. Warfare will cease to be finite. The distinction between war and peace will dissolve. Armed conflict will not be confined by national frontiers. Local belligerents will mobilize foreign patrons.