The TPM is part of the motherboard, not the disk. If someone gains access to the disk, but not the computer, they will not have access to the TPM, which contains the keys.
During boot, the TPM is unsealed by software. The TPM should not unseal, unless the boot environment matches some parameters:
Computers that incorporate a TPM can also create a key that has not only been wrapped, but is also tied to certain platform measurements. This type of key can be unwrapped only when those platform measurements have the same values that they had when the key was created. This process is referred to as “sealing the key to the TPM.” Decrypting the key is called unsealing. The TPM can also seal and unseal data that is generated outside the TPM. With this sealed key and software, such as BitLocker Drive Encryption, you can lock data until specific hardware or software conditions are met.
What this means is that the TPM should not unseal if you try to boot some other OS, or modify the windows binaries that you boot. In this case, the TPM should refuse to unseal, and thus rendering the key inaccessible.
This of course relies on the Windows authentication being secure, and that no security problems are present in the pre-logon environment. This may or may not be true.
In addition, you can require a pin code or password, which will improve security quite a lot, as the pin code or password will be required in addition to the TPM if configured correctly.
Disk encrypted with TrueCrypt/VeraCrypt. Key isn't stored anywhere and bootloader asks for key upon each and every boot. Someone steals my whole computer (or I am loosing it somewhere) and has no access to my data due to fact that it is encrypted with key, they don't know, and providing this key is necessary during each boot.
This is comparable to Bitlocker, with PIN or password required. PIN is used to unseal TPM, and you have a limited number of tries. Secure boot chain ensures that an attacker is unable to modify the binary requesting the PIN, to save it or transmit it.
Disk encrypted with BitLocker. Key stored within TPM module on mainboard and bootloader never asks for it during boot. Someone steals my whole computer (or it is lost) and has a full, uninterrupted access to my data due to fact that it is decrypted on-the-fly during boot.
They still have to bypass the windows login security. On a fully patched Windows with reasonable security policies (e.g. good passwords, DMA on Firewire disabled etc) it's reasonably secure. Porbably not NSA-secure, but almost certainly random theft-secure.
The secure boot chain ensures that the attacker is unable to modify any part of the OS, or boot with a different OS. In this situation, the security hinges on the security of the OS. It's somewhat similar to an attacker that gains access to a locked, but booted machine with Veracrypt or similar.
Bitlocker is fairly configurable. It can use the TPM, or it can not use the TPM. It can require a key stored on a USB medium, or it can require TPM + pin, or only TPM.
In short, security is never absolute. It's all about trade-off between security and convenience. Who are you protecting against, and how much hassle is it worth to protect yourself? You could keep your computer unconnected to the net, in a locked vault in your cellar. That would be inconvenient, but even an unencrypted device would be very safe against random theft. It would obviously not be secure at all against cops with a warrant.