Because in Scala (as in any Turing complete language) there is no guarantee that a Function is total.
val f = {x : Int => 1 / x}
That function is not defined at 0. A PartialFunction is just a Function that promises to tell you where it’s not defined. Still, Scala makes it easy enough to do what you want
def func2Partial[A,R](f : A => R) : PartialFunction[A,R] = {case x => f(x)}
val pf : PartialFunction[Int, String] = {case 1 => "one"}
val g = pf orElse func2Partial{_ : Int => "default"}
scala> g(1)
res0: String = one
scala> g(2)
res1: String = default
If you prefer, you can make func2Partial implicit.