Normally full disk encryption and booting security is the province of your OS. To my recollection both Bitlocker and LUKS can optionally use the TPM which means

Normally full disk encryption and booting security is the province of your OS. To my recollection both Bitlocker and LUKS can optionally use the TPM which means it can be used for 100% of the use cases which would improve computer security. What are we missing out on exactly?