A lot of drivers for hardware are actually not open source, just unreadable binaries that do …something. No one knows exactly how they work, so some people consider them a security risk.
While I do understand the security aspect of this here at the same time those people seem to be delusional. At some point there’s proprietary stuff in our computers, be it a driver, a BIOS or the code that runs on the various microcontrollers that run low level functions from the USB ports to simple power management.
The most “security paranoid” organizations in the world usually run a lot of stuff on Windows and HP hardware full of opaque and proprietary code and they consider it “safe enough”.
I may get that not free / license based stuff might raise concerns if you aren’t a mega corp. that can pay the fee either way, but… if a trackpad requires a free but closed-source binary driver why would a random guy on the internet consider that to be a risk?