Rear is much faster when your hand is already holding the phone. Having to move your thumb to the bottom of the phone for a front sensor not only takes longer but also increases the chance of it slipping out of your hand (since you're only holding the phone at the bottom and not the middle).
And yes, I store my phone in my pocket, so taking it out of my pocket is a perfectly normal thing to do.
What? You still have to put your finger in the correct position if the sensor were on the back, and there are scenarios where your finger wouldn't naturally go to that spot. Meanwhile there is a 100% percent rate of putting your finger on the front of the phone when you use it and you can clearly see where everything is.