Because computers are not designed for private keys.
Discussion
Can't these wallets be hacked?
Probably, but ColdCards are closed source so we really don't know.
Every byte of code can be found here, reproducible too https://github.com/COLDCARD/firmware
Maybe you could move to this license 
to prove you don't want to be a FOSS affinity scammer switch to this license or fuck off
It's million times harder than a general-purpose device.