Because we as a society are not the same as the generations of the gemara. Our kavonos are not the same, our madreiga is not the same, and so what worked then does not necessarily work now.
Without commenting about whether the current system is flawed, and how it can be be improved upon, we need to find a system that is appropriate for now and here. Just as our chinuch system has “evolved” with time – and with the support our current rabbanim- and does not conform to the way children and men were taught in the time of the gemara, so too the shidduch system.
Besides, I don’t think that the proposal is meant as an end to the shadchan, or as a way for boys to be together with women, as you say. It’s purpose is to jump-start the shidduch, get people to meet who might not ordinarily meet, and not for random mingling and breaking down of barriers between boys/girls. The question is how to achieve the first goal in an effective, productive manner, without the second happening as a consequence, which I think is what worries people about the proposal.