Because if you ask me, the concepts are used for something very different:
KPIs are your company’s vital signs. While a doctor might take your pulse and body temperature, you might track revenue growth and profit. KPIs do not direct your action but give you insights.
I like to think that there are two kinds of KPIs: Those that track your business objective success and those that track your business' health. Business objectives are quite clear, so here's what business health means: KPIs that you are not willing to compromise or are essential for achieving the set business objectives.
→ You use them to make data-driven decisions.
That’s where OKRs (or any other objective framework) come into play. If the former communicated vital signs, OKRs would communicate the intended treatment—your plan to move the needle. KPIs show you the data on where to focus time and effort. OKRs outline your approach to making the change.
→ You use them to prioritise your day-to-day action on a week-to-week cadence.
Both have their purpose. And any team should utilise both*.
*At least at the principle level. Language and frameworks are always OK to differ. Principles are few, but you can successfully select your methods if you understand them.