As you say, it's just a matter of familiarity. Sounds like you just need exposure to more programming languages/paradigms, as it's a pretty common convention. I believe it comes from the ML family. OCaml[0] springs to mind. As others have mentioned, Rust[1], Kotlin[2], Haxe[3], Nim[4], Swift[5], Elm[6], Dhall[7], and Scala[8] all use this convention.
The current syntax seems better for type inference and gradual typing.
[0] https://learnxinyminutes.com/docs/ocaml/
[1] https://learnxinyminutes.com/docs/rust/
[2] https://learnxinyminutes.com/docs/kotlin/
[3] https://learnxinyminutes.com/docs/haxe/
[4] https://learnxinyminutes.com/docs/nim/
[5] https://learnxinyminutes.com/docs/swift/
[6] https://learnxinyminutes.com/docs/elm/