The disk encryption key(s) isn't typically stored in the TPM, it is sealed by the TPM. A sealed key can only be unsealed if the TPM is in the same state as it was sealed against (simplifying some things here). With the proper selection of Platform Configuration Registers (PCRs), this can prevent the key from being unsealed if the system has not securely booted (sealed against PCR 7). In more complicated configurations, it is also possible to seal the key against a particular phase of boot such that it can't be unsealed once userspace is reached.

In your scenario, yes it is bad if the attacker gets root on your computer, but sealing the key with the TPM means they can't retrieve the key itself. Where the TPM helps is preventing the attacker from establishing low level (kernel, bootloader, or firmware) persistence, since modifications of these components would change the TPM PCR measurements and result in a boot failure.

If the attacker is local and they reset the UEFI, the TPM PCRs are now different and as before, the disk encryption keys will not unseal.

It is also generally recommended to use a pin with TPM, which further complicates this scenario for the attacker because the TPM enforces rate limiting. As the other commenter mentioned, the physical access scenario is commonly a stolen laptop situation, where the attacker would not be in communication with the victim and probably wouldn't return the laptop.