It would seem to me that if one wanted to tailor the frequency response; it could be done more specifically and more consistently with an equalizer than with the chance of amplifiers not lining up with impedances of loudspeakers in just the right manner towards that end.
In that loudspeakers have mechanical considerations that amplifiers do not; it seems quite understandable that it would be much more difficult to build loudspeakers with truly flat impedances, than to build amplifiers that can linearly adapt to fluctuating loudspeaker impedances.
Having a linear base line is an important consideration towards the goal of "high fidelity". Even if one wanted to deviate from that goal, amplifiers that can better adjust to loudspeaker impedance fluctuations would be easier to use with the afore mentioned equalizers.