I am reading a paper regarding A/B test continuous monitoring. https://alexdeng.github.io/public/files/continuousMonitoring.pdf
In the abstract part, it states
A crucial problem of Null Hypothesis Statistical Testing (NHST), the backbone of A/B testing methodology, is that experimenters are not allowed to continuously monitor the results and make decisions in real time.
But without explain why this is the case.
So my question is: Why can't we run NHST based A/B testing continuously and make real time decision based on the result?