I'm going to assume the 90% number was simply hyperbole. Because it's trivially false in any number of ways;
Firstly many businesses have never heard of A/B testing, much less apply rigorous application of it to proposed changes.
Secondly many businesses have found their niche and don't change anything. There's a reason "that's not how we do it here" is a cliche.
Thirdly a whole slew of businesses are greater changing things all the time. My supermarket can't seem yo help themselves iterating on product placement in the store.
Blaming testing in general, or A/B testing specifically for some companies being unwilling to change, or iterate, seems to be missing the actual problem.
Frankly, with regard yo web sites and software I'd prefer a little -less- change. I just get used to something and whoops, there's a "redesign" so I can learn it all again.
A/B tests, by definition, test between A and B. It is very likely that neither is the best option.
But how will you find the best option if you don't measure options against each other.