Manufacturers have used the 1x setup to modify the geometry of bikes. They can shorten chainstays and also fit wider tyres without falling foul of legal constraints (there has to be a minimum 6mm gap between chain and tyre for example)
You could fit a front derailleur to any of these frames as you aren’t constrained by those rules but you’d have to be prepared to fit a narrower rear tyre should there be interference. It would have to be a “band on” FD as there’d be no bosses on the frame for direct mount.
When I went 1x I reckon I lost three ratios in range. As it happens I rarely use either the top or bottom cogs with the 1x so I’ve probably only lost one ratio. A 30T chainring with an 11-46 cassette gives a range of ratios from 0.65 to 2.72. A 2x with 36/24 up front and 11-36 out back gives a range of 0.66 to 3.27. So there’s a bit missing from the top end, whether that’s important or not is up to you.