Why aren't there submission fees for submitting papers to conferences?

There is already a strong incentive not to submit a paper that you know has very little chance of acceptance. While a paper is under review for one conference, it may not be submitted to another (according to the standard policies in fields where conference papers are "real" publications). If you submit a paper that is nowhere near ready, you hurt yourself: you block yourself from submitting a more complete version of it, that has a good chance of getting accepted somewhere else, for the entire review period.

Furthermore, many conferences end up rejecting lots of very good papers, due to limited space. What is the rationale for collecting fees from those authors? Authors would absolutely take their papers elsewhere, rather than risk paying hundreds of dollars to get a reasonably good paper rejected.

Finally: authors from less wealthy countries are already "priced out" of conferences to some degree (and have to publish mainly in journals), because of the cost of conference attendance (travel, registration fees). This would certainly make the disparity worse.

One reason is fairness. Some papers are rejected because they just aren't good enough, but others could plausibly have been accepted if the deliberations had by chance gone a little differently. When a reasonable submission is rejected due to bad luck, it feels like adding insult to injury if the author then has the subsidize the registration fees for the accepted papers. (It might be reasonable to expect authors to cover the cost of reviewing the submissions, but that's far less than typical registration fees.)

Another reason is to minimize bureaucratic overhead. Whenever you collect fees, you have to negotiate with authors over who deserves a fee waiver, and you have to deal with unusual situations (authors who can't or won't pay online, chargebacks from disputed transactions, etc.). The pain scales with the number of people involved, and it tends to be extra high if you try anything nonstandard, so there's an incentive not to experiment, especially in ways that increase the number of transactions.

Another consideration is keeping authors happy. In any review process, some authors are going to end up disgruntled. Maybe they actually received low-quality reviews, or maybe it just feels that way to them, but either way they may complain. I'd bet that complaints would increase tenfold if the disgruntled authors had been charged $200 and felt entitled to a corresponding level of service. I certainly wouldn't want to be in charge of dealing with the complaints.

You seem to be under the misapprehension that being willing to "put one's money where one's mouth is" is a measure of the quality of somebody's work. It isn't. There are plenty of rich mediocre people in the world, and plenty of poor smart people.

Also, conferences charging submission fees is an unstable equilibrium. Every conference could attract good papers away from its rivals by charging a lower fee than them, initiating a race back to free submission. This doesn't work for attendance fees, because those pay for actual costs incurred by the organizers, so can't be reduced arbitrarily. Also, one is more committed to attending by the time that attendance fees have to be paid.

The premise of he question is wrong. There are submission fees for conferences.

Abstracts are only processed and available for the session organization by conveners after the payment is completed. Please note that this is a processing charge and not a publishing fee. APCs are not refundable in case of an abstract withdrawal, rejection or double submission. The charges collected cover the cost to process the abstracts whether or not one attends the meeting.

http://www.egu2016.eu/abstract_management/how_to_submit_an_abstract.html

Not many abstracts are rejected at this (or other Copernicus organized) conference, but it can happen.

If they are not charged at your conference it means that the organizers don't want to discourage people from submitting an abstract, but if they know they will get enough applications anyways they can charge a fee.