You can donate to your own private blood bank. I would advise this.
Yes, I agree. But the purpose of autologous donations is almost always in anticipation of a planned surgery or other medical procedure. Your blood is stored at a predetermined facility and prepped (thawed) before in anticipation for the event.
But unforeseen traumatic injuries are, by far, the usual call for blood transfusions, and you're only an unanticipated accident away from needing this. You can't be sure you'll be in the vicinity of your own blood when it happens. And even preserved blood has a shelf life; you'll have to continually donate regularly for yourself to make sure the supply is current and viable.
I'm not dissing your suggestion, just pointing out its limits.