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.