It's quite possible that people feel they lack the technical baggage to cope with typescript's type system (typescript = javascript + types). However, I would argue that the same technical baggage is a prerequisite to be able to architect large-scale javascript front-ends. So, IMHO, in the context of large scale apps typescript or flow are a no-brainer.
The fact that it requires a build step turns out not to matter because everyone uses a babel / webpack pipeline anyway, so all that same complexity is there for regular javascript as well.
Front-end dev usually have a tight feedback loop. You don't want to wait longer then necessary to see your changes on the screen. You only run the build pipeline when you are about to push your updates into production.
The fact that it requires a build step turns out not to matter because everyone uses a babel / webpack pipeline anyway, so all that same complexity is there for regular javascript as well.