TPMs are notified about the system state along the boot process, so they can present the key only if the boot process looks like it's supposed to (and some of t

TPMs are notified about the system state along the boot process, so they can present the key only if the boot process looks like it's supposed to (and some of these attributes aren't even modifiable by the CPU but the side processor, e.g. Intel's "Management" Engine or AMD's "Security" Processor, so even taking over the x86 firmware doesn't help anymore) Smartcards lack that data (they wouldn't know that somebody attached a Thunderbolt device that pretends to be a GPU but also comes with its own option ROM that installed a backdoor), so they serve a different threat model.