Why is there a charge for attending a virtual conference?
A virtual conference organised by a non-academic organisation carries several costs. A non-exhaustive list of these costs includes administrative staff salaries and national insurance; website hosting, design, maintenance, updating; accounts and taxes; business insurance; software licences; equipment, and office space. These are fixed costs and are not reduced whether an event is held virtually or in-person.
There are costs even for the conferences organised in academic institutions, but these costs are generally absorbed by the institutional infrastructure, organising/academic staff salaries or grants from funding bodies.