Speakers are rated in simple Ohm ratings. Most speakers impedance changes across the frequency spectrum. Music signals are a mix of many frequencies at once making for a very complex relationship.
Amplifiers manufacturers give you a simple ohm rating so you do not severely overload the amp. Say by running 2 pairs of 4 ohm speakers in parallel.
Speaker manufacturers give you a simple nominal ohm rating to help you select a suitable amp or which taps (transformer or autoformer) to run their speaker off.