Because I get paid, sometimes, to make speeches. A lot of fortunate journalists and opinion-makers do. Others want to, but their employers either won't let them or impose severe limitations on these lucrative appearances.
The thinking is that if we take the big bucks from the very groups that can afford to pay, our objectivity might be compromised and we might slant our reporting to favor the wealthy special interests. At the very least, we might look like we are.