Why are private jets more expensive to travel in than commercial aircraft?
Aside from the economy of scale effect of a large aircraft vs a small one you can compare apples for apples by looking at the same type used on both roles.
The Canadair CRJ200 was sold as both a regional airliner and a corporate jet, called the Challenger 850, so it makes a good comparative example.
The Challenger 850, being, mechanically, a CRJ200 with a corporate interior, has pretty much the same numbers for fuel burn etc, as a 200.  But it's only carrying 1 to 12 passengers, vs up to 50.  So even &quot;full&quot;, its fuel burn per occupied seat mile is always going to be several times higher than the airliner version operating with decent load factors (and they aren't much lighter; the corporate interior is very heavy and with 12 bodies aboard you are close to gross weight with a large fuel load - add in a long range fuel system option by STC for that airplane, and the payload with full fuel was 2 persons... yes, 2).
Then there's utilization.  The CRJ 200 airliner averages about 2500 hours a year.  A Challenger 850, like most corporate jets, averages 300-400 hours per year (corporate shuttle versions fly a lot more, maybe over 1000, but they have regular airline style interiors).  All the fixed costs of operation are amortized over a way smaller number of operating hours (most corporate airplanes would fall apart if you put airliner hours on them; they are not designed for that level of use - this made the Challenger 850, with an airframe meant for 2500 hours/year, absolutely bulletproof in its corporate role).
So you have a cost double whammy.  You're using several times more fuel to get around for each warm body on board (what I might call &quot;warm-body-miles&quot;), compared to an equivalent airliner, and you're doing it infrequently and so are paying through the nose for all the fixed costs like insurance and hangar space, etc.
The reason for the existence for most corporate airplanes is the time valuation of the &quot;warm bodies&quot; travelling on it.  A large company makes the calculation that the value of the CEO, and the CEO's lieutenants' time, justifies the astronomical operating cost of a corporate aircraft; in effect, the jet makes money for the company by making the senior management's job more efficient (in theory - no doubt a lot of it is just &quot;because they can&quot;).
If you're the other type of corporate jet owner, the ultrawealthy who own the planes personally, economics doesn't even come into it (I knew a guy who looked after corporate jet purchasers pre-delivery, and he told entertaining stories of bizarre customer meetings, with oligarchs and their children arguing over 500,000 dollar interior options as if they were fighting over who got to lick the cake icing spoon).