Because any process with i/o capability on the host OS can read the private key on the USB?
Short of a oscilloscope hiding in the computer you use (totally possible), no process can derive the private key from the Trezor in theory.
Even if you have processes that blatantly copy every USB's contents (or even log all interactions verbosely) and log all key presses/clipboard interactions on the machine, you can still use a Trezor without compromising anything.
You can also verify that your clipboard is not being manipulated as the Trezor can verify the address it will be signing a transaction with on the display.
Short of a oscilloscope hiding in the computer you use (totally possible), no process can derive the private key from the Trezor in theory.
Even if you have processes that blatantly copy every USB's contents (or even log all interactions verbosely) and log all key presses/clipboard interactions on the machine, you can still use a Trezor without compromising anything.
You can also verify that your clipboard is not being manipulated as the Trezor can verify the address it will be signing a transaction with on the display.