It`s not that past times were better or somehow produced better people. Many potentially great future playwrights of the past were probably lost before having even written their first play, because they decided to work in a profession that would earn their living instead of starving to death as a playwright. Talent was never a guarantee for success. Becoming a playwright was much more risky in past times, financially and in times when thought was not so free also risky in terms of censorship and incarcerations. So there weren`t probably so many people who decided to become a playwright because they had “always wanted to do something creative” or “didn`t feel an office job was for them”.
But, yes, each time had its mediocre ones, of course. Not all of them were forgotton, some survived because some nation, desperate to have its own national poets, didn`t have so much of a choice and so they had to choose somebody like Friedrich Schiller as their national theatre playwright, Friedrich Schiller who was a great sitting-on-his-sofa Hollywood plot writer but who had no idea how to write a scene that would function on stage. But many of the mediocre ones were forgotten, as well as the more mediocre plays of the greater playwrights. We simply don`t know them anymore, and we should be thankful for that, even if there might be some errors in the system.
Of today`s theatre playwrights unfortunately no one is already forgotton. There might be some Mozart-Salieri thing going on with the one who is more recognizable being more acclaimed. There is also fashion which has always been the enemy of quality. And talented writers will not search their luck in theatre, as theatre is dying. They will write for netflix and for games. And human progress has brought us so far that even bad theatre playwrights don`t die from hunger. I feel that I should add that this of course a good thing.
But, if you want to get a taste of what a good quality play is, and I think you should get this taste because it will build good quality structures in your brain: Go for the old ones. 100 years is a good distance. And try Friedrich Schiller. He is difficult to do, but I have seen one good scene from his plays in my life, so it is not impossible.