There's a few reasons. The biggest one, IMO, is that it lets non-technical users change things quickly without having to go through the engineering team. Obviously there are limits to that, but in many cases, a product or marketing team wants to modify a form or test a few variations without having to put it into a backlog, wait for engineers to size it, wait for an upcoming sprint, then wait another two weeks for it to get completed and deployed. (Even in more nimble organizations, cutting out the handoff to engineering saves time, eliminates communication issues, and frees up the engineering team to do more valuable work.)
On the technical side, these form builders can actually save a decent amount of development effort. Sure, it's easy to build a basic HTML form, but once you start factoring in things like validation, animations, transitions, conditional routing, error handling, localization, accessibility, and tricky UI like date pickers and fancy dropdowns, making a really polished form is actually a lot of work. You either have to cobble together a bunch of third-party libraries and try to make them play nicely together, or you end up building your own reusable, extensible, modular form library.
It's one of those projects that sounds simple, but scope creep is almost inevitable. Instead of spending your time building things that actually make money, you're spending time on your form library because suddenly you have to show different questions on the next screen based on previous responses. Or you have to handle right-to-left languages like Arabic, and it's not working in Safari on iOS. Or your predecessor failed to do any due diligence before deciding to use a datepicker widget that was maintained by some random guy at a web agency in the Midwest that went out of business five years ago, and now you have to fork it because there's a bug that's impacting your company's biggest client.
Or, instead of all that, you could just pay Typeform a fraction of the salary for one engineer and never have to think about those things ever again.