Club site pricing

 I would like to know why club sites charge per person,

I have just got my sister & her husband into Caravanning, they have 5 children, when looking at sites to stay on & working out the price per night for her family it works out to be silly money per night, why does the caravan club charge for extra people, the pitch is paid for, so looking at it this way

1 pitch will take a 6 berth caravan, that will sleep 6 people on the same pitch. without an awning.

so why do we have to pay for extra people to stay on the same pitch, in the same caravan ?

she was told she would have to pay for a 6 week old baby ?

so to stay at ferry meadows CC site in May for my sisters family will be :-

3 Adults @ £7.70 = £23.10

1 Pitch @ £9.50 = £9.50

3 Kids @ £2.70 = £8.10

that works out to £40.70 PER NIGHT

For my wife & I for the same pitch in may will be :-

2 Adults @ £7.70 = £15.40

1 Pitch @ £9.50

that works out to £24.90 PER NIGHT.

WHY ? they don't take up anymore room than my wife & I do !