I've got (virtual) servers all over the place. I run my tiny little "needs a minute" tasks on one or more of those. It's nice, I guess to make it a wholly isolated thing, but I rarely actually want to go to the trouble to design it to be isolated. My little jobs often need at least some of the rest of my production environment to be available (database, logs, libs, something). Making a Lambda thing just for those things seems like something I'd rarely want to go to extra effort for.

But, if building on a very large scale with strict isolation requirements for your individual tasks it would make sense. Which is the niche I'm talking about; maybe it's just bigger than I assume it is.