There's not a lot from a electrical engineering perspective inside a hardware like the Trezor. It's a small microcontroller, a very cheap OLED screen, some miscellaneous buttons and connectors. The entire bill of materials is likely under $10 USD for per unit.

The difficulty comes from the software and integrity perspective. You've got to ensure that the software doesn't lose other people's money, that it protects in the ways it intends to, and resists tampering. The hardware should be able to be produced with some confidence that the software written to the devices is secure, that nobody can replace them in transit with identical, but malicious devices. You've got to be sure nobody engineering the devices has their own backdoors to steal money from the device, that there's not ghost runs being made by a producer.

If there's a large theft relating to hardware you've produced, you'll be the one immediately blamed as responsible for backdooring them, so this is an existential threat for the producer if they get it wrong. Are you willing to take this personal risk? Most people would not agree the benefits outweigh the costs, if producing secure hardware seems like a easy idea you've probably not considered all the modes of failure.