What's infuriating is that USB drives used to come with hardware write switches and now you can't find them anywhere. And motherboards used to require you to move a jumper to flash the BIOS but, those are gone too. I don't know if it was cost cutting or a case of user stupidity or both but, the hardware write switch has faded into history. I'm fine with the being in a default-write setup as long as they had the option to cut it off.
A third possibility is that the NSA and their friends abroad might have pressured the manufacturers to remove these security features. The pressure might have subtle, like pointing out "good" places for cost savings.
As someone else mentioned, Kanguru has write protect (and I think a few others -- I have some drives by Imation and RiData that have the switch). But that doesn't necessarily protect you from something like badUSB, which can infect drive firmware.
Kanguru states their drive firmwares are protected with digital signatures. However, that means the firmwares are writeable under certain conditions, and we now know that certain organizations make it their job to steal the private keys of security vendors (you can bet the practice is not limited to SIM cards). In that case, you may be better off with a 'plain' thumb drive that has a non-changeable firmware especially if it has a write-protect switch.
What really, really sucks is that virtually no manufacturers are stepping up to the plate with better hardware designs that can mitigate the problem... and even the OPAL2 spec appears to state that firmware protection is optional. Merely putting write-protect jumpers on the firmware storage chips would prevent most attacks (the remote ones).
An exception to the lack of manufacturer concern may be the new Purism brand that just launched their Librem 15 OSS-friendly laptop. They are interested in putting at least a jumper on the motherboard that can block BIOS changes. They also promise to release an edition of the Librem that allows the user to cut power to wireless, mic and camera.
Another mitigation is Qubes OS, which has an architecture that greatly ups the bar for security and it can detect tampering in the BIOS, kernel, hypervisor, etc.