Batteries (or rather, individual cells) do have a standard: 18650. Unfortunately too thick for the ultra-thin laptops, but the older Thinkpads use them. I suspect safety is the reason why no one makes replacement laptop battery "empty shells" that take 18650s and have the appropriate balancing/protection circuitry to interface with a laptop, but then again you see mobile phone powerbanks being sold this way... go figure:

https://megaeshop.pk/5v-dual-usb-power-bank-box-diy-shell-ca...