"Customization" increases costs.
It is easier and cheaper to design one then sell multiple units than to custom design for each client for a single sale.
Trying to design speakers to match each customer's "subjective" tastes is NOT an easy task - how do you know what I like? And you have to do it for each customer. There will likely be a lot of trial and error until meets customer's tastes - this is a lot of time and effort for one customer. What if the customer refuses to be satisfied with the results, will the manufacturer have to eat the time and effort?
No @kenjit you'll find few customers paying a speaker manufacturer substantial money to get a speaker manufacturer to create a "subjectively" ideal speaker - it's easier and cheaper to buy ready made speakers.
And no, @kenjit speaker manufacturers have no incentive to do this extensive custom work per each customer - more work and grief for lower profit (customization may increase profit markup per speaker, but less total number speakers will be sold)