While they're building, yes there are absolutely people on call for issues on the site out of hours.
At my former employer I was on an on-call rotation; I'm obviously not now it's a 'former' employer, so the building analogy doesn't really hold up. (And not just leaving the company, but e.g. my former colleagues now working on something else at the same company aren't on call for the software they wrote but are no longer responsible for.)
Devs think they're hot stuff, when in reality we're probably one of the most abused professions out there. (I'm talking about regular devs, not people who were born in wealth/went to good schools etc)