Because every version of flutter_localizations from sdk depends on
Adding this to pubspec.yaml solved it for me:
dependency_overrides:
intl: 0.17.0-nullsafety.2
This is versioning issue. I would advice you to update your Flutter SDK
(from flutter_localizations from sdk depends on intl 0.16.0 is visible
that you are using some of the old versions, because last versions are
compatible with intl 0.16.1. If you don't want to update it, you can use
another lower version of flutter_form_builder < 3.7.3 which will be compatible
with intl 0.16.0