Because if you print it, you know you have it. You can never tell what they will ask for. Yes, you don't have to, but when using a computer instead of a phone, it is just one button. Phones have been known not to connect or run out of batteries. I just printed one out a few minutes ago and stapled it to my ticket, just because.

This is how it was for me.  I had the TDAC on my phone ready to go and was never asked for it.  I asked the officer if he wanted to see it and he said it was on his system so no need for me to show. Maybe if you filled it out at the last minute it might not show up on their side?  Don't know, just a guess.

think I have been the unlucky one with this then I forgot to hand it over with my passport and boarding card, and got asked for. 
Think it is a case of if you get an immigration officer who wants to do his job properly,  but by the sound of it most just want to get you into the country and can not be bothered. 

I have been 3 times since it was introduced. Haven’t been asked to produce it yet……as you say, will be on the system when they log your passport at immigration.

Copy that ... for my last two visits, I filled it out a few days upfront and didn't get bothered by it when passing immigrations