Why the declaring types drives GHC crazy?

Applicative.hs

lift3 ::
  Applicative f =>
  (a -> b -> c -> d)
  -> f a
  -> f b
  -> f c
  -> f d
lift3 f a b c = 
  {-let  -- This produces the error below
    g :: f (c -> d)
    g = lift2 f a b
  in-}
    lift2 f a b <*> c 
    
{-
    • Couldn't match type ‘c’ with ‘c1’
      ‘c’ is a rigid type variable bound by
        the type signature for:
          lift3 :: forall (f :: * -> *) a b c d.
                   Applicative f =>
                   (a -> b -> c -> d) -> f a -> f b -> f c -> f d
        at src/Course/Applicative.hs:(189,1)-(195,8)
      ‘c1’ is a rigid type variable bound by
        the type signature for:
          g :: forall (f1 :: * -> *) c1 d1. f1 (c1 -> d1)
        at src/Course/Applicative.hs:199:5-19
      Expected type: f1 (c1 -> d1)
        Actual type: f (c -> d)
    • In the expression: lift2 f a b
      In an equation for ‘g’: g = lift2 f a b
      In the expression:
        let
          g :: f (c -> d)
          g = lift2 f a b
        in g <*> c
    • Relevant bindings include
        g :: f1 (c1 -> d1) (bound at src/Course/Applicative.hs:200:5)
        c :: f c (bound at src/Course/Applicative.hs:197:13)
        f :: a -> b -> c -> d (bound at src/Course/Applicative.hs:197:7)
        lift3 :: (a -> b -> c -> d) -> f a -> f b -> f c -> f d
          (bound at src/Course/Applicative.hs:197:1)
    |
200 |     g = lift2 f a b
    |         ^^^^^^^^^^^
Failed, 12 modules loaded.

-}