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.