I think it's "weird" because it fills this weird gap between shitty macros and generic programming but does none of the hard parts of either. The syntax itself is not nearly as strange as those semantics.

For example, if you look at what type traits are used for, it's essentially making up for the fact that C++ lacked semantics for compile-time checks of generic arguments, forcing anyone that needed to care about that to implement their own (often simple, special case) constraint solvers. Meanwhile, typeclasses in other languages wrote a generic constraint solver (ironic) and support the semantics to express those constraints on generic types, obviating the need for any kind of complex compile-time template logic hackery. Essentially the lack of semantics for a simple concept (yet difficult implementation behind it) forced the implementation of a simple extension to the same syntax that enabled really complex macro programming using the same templating engines.

It's no surprise that metaprogramming in C++ is exceptionally weird; it supports neither proper macros or generic programming, yet half-implemented both through templates to solve the lack of either.