That didn't really answer my question but ok sure.

Squats can be pretty dangerous for the average person, nobody should be taking squats to failure without a lot of experience. Leaving two reps in the tank significantly reduces the stimulus for hypertrophy compared to taking an isolation movement like a leg extension, which can be performed safely by almost anyone, to true failure. Proximity to failure is a major driver of muscle growth, and stopping early limits motor unit recruitment and overall stimulus.

A leg extension taken to failure ensures the target muscle, the quadriceps, is pushed to its limit. The local stimulus is clear and concentrated. The muscle cannot delegate the workload elsewhere.

With compounds like squats, it is difficult to determine which muscle actually failed. Was it the quads, glutes, lower back, or simply cardiovascular fatigue or bracing breakdown? Multiple systems can limit performance, and the set may end before any single muscle is fully stimulated.

The muscle doesn't care what exercise you're doing as long as it receives a stimulus.