I don't know anything about Trusted Signing verification, but I do know from reports on 'mini umbrella company fraud' that if you're a fraudster, there are people in the Philippines who will happily sign their name to western countries' official paperwork in exchange for $2000 or so. Understandably, as that's more than the country's median annual income.
So I can see why offering trusted signing for individuals worldwide would come with certain challenges.
Most RATs are signed, that's a hurdle but it's clearly not a big deal to bypass for criminals, many "SSL companies" provide them, just have to use fake docs and you'll be issued it, many shady services sell those signatures as well and it doesn't look like it cost more than $15 per binary, so obviously, not so secure in practice.
So I can see why offering trusted signing for individuals worldwide would come with certain challenges.