I detailed why that's not acceptable. You forget that you made the purchase because it's not in the stack of other receipts, and second, some companies don't accept web printed receipts without further justification, especially since you can buy alcohol on board.
At minimum, I have to go back after the trip, figure out the flight number, date, and credit card, then print it out, instead of just handing the stack of receipts to the travel people. It's even bad for the environment: instead of a small strip of paper, the receipt is now 8.5x11.
The old system used a handheld device that had a printer built in. I'm not the first person to complain:
https://unitedairtime.com/your-questions/on-board-purchases-are-paperless/