"why you build software" = requirement.
end-user/customer/client happiness = Customer satisfaction surveys and ratings help you acquiring it. Basic KPIs such as retention rates and engagement can also help you understand this.
A/B testing helps the process of testing assumptions in a data driven way.
That is a definition of requirement that is different than what most of the industry uses. Most of the industry sees requirements as "how the software should behave". Even with your new definition, I've seen little evidence that we have mature processes around collecting requirements.
As for surveys, KPIs, A/B testing etc. Those are neither standardized, proven to be valuable in a rigorous way, nor widely used. All signs of an immature industry. Further, they can only be used in software development processes that allow for iterative deployment.
A/B testing helps the process of testing assumptions in a data driven way.