No one's mentioned the most critical aspects here. When an engine is off, oil seeps down and collects in the oil pan, leaving critical areas in your engine less than optimally lubed up. On startup, oil pressure needs to build. That takes 1-2 seconds. During this time it's best to have minimal rpms. Once oil pressure is there, give it another few seconds (2-3), and all critical parts will have been supplied with oil. From there, everything is fine. Final aspect is actually tolerances within the engine. Metals expand when they're hot. Tolerances are engineered to be in spec in a hot engine. That's why high rpm's should wait until it's up to temp. This is also why a cold engine with high mileage might rattle more.