because the suspension on the back lets you away with things so you dont learn basic bike control skills. Like how to move your body to absorb the rough stuff. How to pick smooth lines. Loads of other skills can be learnt by riding a hardtail. You'll also appreciate the full sus more when you move on and get better use out of it.
There's no such thing as stupid question, unless it is "Will my Panaracers remain puncture free?"
The simple answer is that a HT demands that you pay attention to your line, that means reading the terrain, which means you learn to be at the right speed at the right time, and you learn about balance, shifting weight, when to apply power, etc. In short you become a technically skilled rider, and people respect that far more than speed or courage.
A FS will let away with a lot more, which usually means that you become lazy about riding around stuff and don't get as much feedback from the ground. So you go to fast to soon and end up binning yourself, because you haven't the skill set to deal with things when the become tricky.
The best way to enjoy a FS is to become skilled on a HT.