fuel.
For instance.. a 10 hour flight might use 35,000 gallons of fuel and IIRC, a plane needs 1.5x the estimated fuel in order to take off.

So... even if you're only traveling with 6 people, you need a plane that carries 50,000 gallons of fuel, meaning that if you want to fly without refueling you need a 747. The cost to fly a 747 is about $25,000 per hour.

Years ago I priced out what it would cost me to fly SLC to NYC private each year vs 1st class on DL. The math worked out that for every $10k I'd spend on 1st class tickets on Delta it would cost me $1M to fly private. So Delta it is (lol).

But... if you fly a lot and most of it is regional... the private by the hour route can be appealing.