You can actually do this, as a fob, but you have to buy one from a vending machine in the gym. Remembering the pin is easier.
That’s also really bad. Who wants to carry a wristband around everywhere? Keychain barcode tag works fine.
What do you mean everywhere? You just throw it into your gym bag.
Many gyms let you rent lockers by the month, so you can store your gym bag there and be able to drop by without necessarily having planned on it when you left home. Can’t do that if you have to carry a wristband around to get in.