Why is std::unique_ptr::reset() always noexcept?

The requirements of the call to the function object Deleter are specific on this as listed in the requirements of the std::unique_ptr<T>::reset() member.

From [unique.ptr.single.modifiers]/3, circa N4660 §23.11.1.2.5/3;

unique_ptr modifiers

void reset(pointer p = pointer()) noexcept;

Requires: The expression get_deleter()(get()) shall be well formed, shall have well-defined behavior, and shall not throw exceptions.

In general the type would need to be destructible. And as per the cppreference on the C++ concept Destructible, the standard lists this under the table in [utility.arg.requirements]/2, §20.5.3.1 (emphasis mine);

Destructible requirements

u.~T() All resources owned by u are reclaimed, no exception is propagated.

Also note the general library requirements for replacement functions; [res.on.functions]/2.