there are 3 contractors who earn more than double what I do (being paid by the hour).
I don't know about your country, but it is probably similar.
In the UK, computer consultants, would get about double the hourly rate, of a permanent employee. But that is correct, and means that in real terms, they are getting a comparable salary.
This is because they are getting ZERO, holiday pay, sick pay, pension and have to pay various extra charges, which your employee is paying for you. They usually get NO employee benefits, either.
Their operating costs (to themselves), are usually considerably higher, because typically they have to drive considerably longer journeys to get to work (as it is a temporary job). They have to pay accountancy fees, and spend time working out the taxes. They may even have to pay for a hotel during the week, or pay rent on a flat. In London, this can get quite expensive.
All extra activities, such as training courses, will NOT pay them any money, and they will have to fork out for the expensive courses.
They usually don't get any employee benefits, either.
Pensions can be quite expensive to pay for.
In the UK, there are taxes which the employer pays. If you are a contractor, then YOU have to pay these taxes (Employer National Insurance), in addition to the taxes that you pay on the salary as well.
The consultant has to allow for the fact that they can be dismissed/terminated, at very short notice. E.g. 1 month.
The consultant could be out of work, during quiet periods, in the computer/software industry. This has to come out of their own pocket.
By the time you add up all these "hidden" costs. There is much less difference in real terms, between the salaries.
Occasionally, because of sudden shortages of employees etc. E.g. The Year 2000 millennium (date) bug. It can be very lucrative, despite what I just said above, for the consultants, As the hourly rates, can go up and up.
There can be considerably more job stress, when you are a consultant. Partly because there is no job security at that place of work. Unlike the permanent employees.
EDIT: There are probably other costs involved, as well. A big one, I forgot, is that they are usually done through agencies, who take a rather significant percentage of the payment, for themselves. When the consultant is told the hourly rate, it is usually AFTER the agency has taken their cut.