Would first encrypting a password make slow hashing algorithms unnecessary?
Yes this would make brute-forcing more expensive, but as was pointed out by “kelalaka” it creates an additional key management problem.
The more classic way to make brute-force more expensive is by iteration. A hash of a hash of a hash … of a hash a few hundred thousand times can be made arbitrarily slow without creating additional key management problems.
Fundamentally the expense to generate is what makes it expensive to brute-force, all other things being equal.
An unequal example would be a too small hash space. If I assume an arbitrarily complex hash-encryption-magic method that produces a 32-bit hash result, I only have to find a password among 4 billion, and it’s probably not even the one you used but it will still work because of the small collision space.