They understand the usage stats on Hidden Photos pretty well I imagine. So much so that they introduced it almost certainly because of the kinds of apps available that do something similar. I don't think it's reluctance on their part to take it further so much as Apple typically doesn't release 500 features at once so much as they dribble them out year after year. However, to first offer hidden photos, then later hide the folder image but then not secure it with the passcode or Touch/FaceID doesn't make sense. You either want it locked down or you don't. You'd think Apple engineers themselves would want it because they without a doubt carry information about projects no one else is supposed to see on their devices and understand the importance of being able to keep it under an extra layer of security.