Why is a network that deploys only IDS particularly vulnerable to an atomic attack?

Why is a network that deploys only IDS particularly vulnerable to an atomic attack?

• The IDS must track the three-way handshake of established TCP connections.
• The IDS must track the three-way handshake of established UDP connections.
• The IDS permits malicious single packets into the network.
• The IDS requires significant router resources to maintain the event horizon.
• The stateful properties of atomic attacks usually require the IDS to have several pieces of data to match an attack signature.

Exam with this question: CCNA Security Chapter 5 Exam (CCNAS v1.2)