Here’s a thought experiment for you:
Imagine that there was a sport where all you did was single arm bicep curls to power a machine along a track. Athletes competed over different distances just like in cycling from all out sprints to multi-hour endurance distances.
Would this be fundamentally different to cycling?
How would the body react to lactate production?
Would there be a lactate stead state or FTP?
How hard would the respiratory and circulatory systems be working at FTP?
If the answers to these questions mean that FTP produces a lower heart rate than it would with more muscle recruitment then I think that would be the fundamental reason for different zones for different sports.