Here's why:
* String is often used individually: We usually talk about "a string" or "strings" rather than a group of them.
* Specific uses have other terms: When string is used in a specific context, other terms are more common. For example, you might say a "bundle" of string, a "ball" of yarn, or a "skein" of thread.
If you need to describe a group of string, you could use a general term like:
* Collection of string
* Assortment of string
* Pile of string
* Heap of string
Ultimately, the best term will depend on the specific context and what you want to emphasize.