Forms are hard because they are the most stateful and most ubiquitous UI component that everyone comes in contact with. HTML has a few barebones tools to help you out, but they aren't good enough for the best user experience if you really wanted to polish a form like show errors exactly when you want to show them and only allow submission exactly when you want to allow it and represent error states in much better ways.

It's especially obvious once you're making forms outside of HTML and you realize that it's not any simpler with any less UX consideration. The only thing that changed was the language you're writing that polish in.