LO platforms use composite materials because they do not reflect radar and are:

Composite materials are chosen for low observable platforms primarily due to their radar-absorbent properties and structural characteristics. In the context of stealth technology, these materials are often nonconductive, which means they do not interfere with the radar-absorbent coatings and materials used in conjunction with them. Nonconductivity helps maintain the effectiveness of radar-absorbing treatments because conductive materials might reflect radar signals, negating the low observability benefits sought in stealth design.

The use of nonconductive composites ensures that the aircraft's radar cross-section is minimized, enhancing its stealth capabilities. This aligns with the fundamental goal of low observable aircraft to avoid detection by enemy radar systems. While lightweight materials are important for performance and fuel efficiency, the distinct advantage of being nonconductive in the context of radar reflection is crucial to maintaining the low observable characteristic of these platforms.