Why cant you charge a pair of bluetooth headphones and use them at the same time?

my headphones died so i put them on charge but that means i can't use them now. i understand they wont be usable for the first few mins cos they are dead. but once they get a bit of charge, why can't i keep them plugged in while im using them?

this applies to my headphones i used for work (bluetooth, connected to laptop) and my own headphones (bluetooth, connected to my phone)