I've never been to a PureGym; if you guys use a PIN-pad to enter does that mean they're like those unattended 24/7 gyms?

...or if they do have an attendant there, why can't they let you in with a friendly greeting like they used to in some imagined past?

They're 24/7. There are usually some staff onsite during the day, but all the entry/exit stuff is always through the automated gates.