Because I have my phone in my hand quite a lot, but I don't have my credit card in my hand at all. To use my credit card I'd have to get my wallet out, find th

Because I have my phone in my hand quite a lot, but I don't have my credit card in my hand at all. To use my credit card I'd have to get my wallet out, find the right card, drag it out of its slot, etc. To use my phone I reach into my pocket, grab the only phone-shaped thing in there, and while moving it towards the terminal put my thumb on the sensor. Takes far less time.