I believe that one reason* many view these as separate abilities when they really aren’t is because of the shallow slope of the exercise intensity-duration relationship. It’s a lot easier to detect, say, a 15% increase in time-to-failure than a 2% (<5 W for many people) increase in FTP, even though they are exactly the same thing.
*Of course, the A#1 reason is overestimation of FTP due to 1) use of suboptimal tests, and 2) ego.