> You have to keep the types in your head anyway so it just increases the mental burden when reading the code.

It's the opposite! Type inference means you can rely on the compiler to check that everything is consistent while you just read what's in front of you. You don't need to think about types at all; the annotations would just be noise.

It simplifies reading (via not needing to care) and writing (via suggestions/auto-complete/jump-to-definition/better error messages).