Why does little endian apply to numbers and not to text strings?
The premise is wrong. Unicode encodings include UTF-16BE, UTF-16LE, UTF-32BE and UTF32-LE. Only UTF-8 has no Litte-Endian or Big-Endian variants.

Fundamentally, Endian-ness is about the byte order of multi-byte words, and you're thinking of text encodings which use a single byte per character (i.e. ASCII). There's no order to a single byte.