Because no cooking equipment can deliver energy at a known precise rate. That means you cannot say "t=0 start frying meat. t=300s add liquid" because you migh

Because no cooking equipment can deliver energy at a known precise rate. That means you cannot say "t=0 start frying meat. t=300s add liquid" because you might cook the meat faster or slower depending on equipment and what shape and size the meat was cut into. Most recipes I have read are written as a less precise timeline though with exact times traded for estimates relative to the previous step or "until done"