Fun, collective work and love of mathematics’ for mathematics’ sake?

The group also got its start in the 1930s, the current “publish or perish” mindset was decades away. And new subfields were still getting discovered left and right so there was no lack of material anyway.

It was done for the advancement of mathematics, not something so trifling as an individual's career. Explaining this makes me feel incredibly old.

People, including mathematicians, have been career driven for a lot longer than we have been around.

I don't think that anything published under Bourbaki's name is considered research work, though.

Exactly. I think the goal was to formalize and codify mathematics. They didn't publish any original papers.

They published a couple of original papers: https://en.wikipedia.org/wiki/Nicolas_Bourbaki#Articles

I assume they were lacunae in the literature that came up while writing the books, but Wikipedia doesn't explain.

Most if not all of them were university teachers in France and former students of ENS (so they already were civil servants before their career even started). Their careers weren't really at stake.

Most of the members had very impressive career anyway. Obviously there are no Fields medalist amongst the founders - they were already too old but I think something like five of the later members got it (Schwartz, Serre, Grothendieck, Connes and Yoccoz). There are probably more if we could count the current members.