I agree overall that many things suited to libraries have become SaaS, but Zapier and similar are well-suited to being a SaaS. For example, a lot of Zapier's services are triggered by incoming webhooks, which doesn't work so great running on the boss's desktop. Same with cron-triggered services.

You really need to run these kinds of services on a server. And the people who use Zapier typically aren't comfortable setting up a server.

Also, there are some very well-done and rather old open source alternatives to Zapier (Huginn, Node Red, etc) but they run best on servers for the reasons mentioned above, so few non-technical people use them.