If it wasn't so transmissible, it wouldn't have been a pandemic.

The question was not "why was it a pandemic", yhe question is, why was it so transmissible when earlier outbreaks, like SARS, had relatively much poorer transmission? That's the typical profile of new viruses.

Spontaneous mutations mean that a virus can become more infectious by bad luck alone. We were unlucky.

The virus becoming more infectious over time is exactly my point. That's typical. What's not typical is the virus already being so infectious right from the start. Normally a zoonotic transfer circulates poorly in the human population before it mutates to become more infectious for the host. COVID-19 was already excellent at infecting humans from the earliest points we've found. That's very, very unusual.