Why does. The water not get hot if the knob is turned all the way to "hot" when first turning on water? Is this a safety mechanism? Can we turn it off? If knob is turned 3/4 to hot, plenty of hot water so it can't be the hot water heater

There is a fair amount of water that stays in the water line between the water heater and the faucet or shower. That water cools down and has to be flushed out before the hot water flows all the way through.

Pretty much agree with cfrias, most taps take a minute to get fully hot depending on the distance from the tap to the heater.