The people who signed my keys trust me to be an honest human actor that chose this as the singular identity they signed for the human body they met in person.

I -could- burn my 16+ years of reputation by letting a bot start signing commits as me, and I could also set my house on fire. I have very strong incentive not to do so as my aggregate trust is very expensive and the humans that signed me would be unlikely to sign a second if I ruined the reputation of my first.

This incentive structure is why web of trust actually works pretty well, and is the best "proof of human" we are likely ever going to have while respecting privacy and anonymity for those that need it.