That's by design not mistake. Within the Schengen Area borders are pretty much left open. Even flying you might be asked for ID by the airline but won't be by border control. If you drive from France to, say, Belgium, you'll just drive past a "Welcome to Belgium" sign.

Indeed. I've done many a road trip around Europe and the only time I've ever had my passport checked inside the EU was entering and leaving the UK.

> Within the Schengen Area borders

Well, it's the same AT the Schengen border these days. It's not a border, it's a joke if anyone can go past it.