ripgrep always deals with UTF-8. When it sees a different encoding, like UTF-16, ripgrep first transcodes to UTF-8 and then searches.
This is absolutely in part because of all of the byte oriented optimizations that are baked into ripgrep (and its regex engine). Note that I said a part. Making ripgrep (and its regex engine) work on things other than a sequence of bytes is far more difficult than just porting a bunch of SIMD algorithms. There are also many optimizations and architectural constraints in the code based on the alphabet size. That is, with 8-bit integers, its alphabet size is 256. With 16-bit integers, the alphabet size is 65,536.
I think this is the right choice because in practice UTF-8 "won" just like how the two's complement machine integer won. It's pretty good, Wikipedia has a brief section explaining how Ken Thompson for example made it self-synchronizing, which seems like a "duh" feature today but the concept before Ken touched it didn't have this. It's a Best Common Practice for the Internet, it's the default in most modern systems and places such as Java's virtual machine or Windows which can't easily "just" use UTF-8 have nevertheless gradually shifted toward being very friendly toward it.