The Babe was the right man in the right place at the right time. I don't think he would be the same person if he played today, and if you just time warped 1920s Ruth to now and put him in an MLB game, he would probably strike out each time up and get sent down to the minors.
I like this clip comparing Olympic gymnast gold medalist performances in 1932 and today, which I think sums up the problem:
https://www.youtube.com/watch?v=QmyoC6bYbmE
The game and standards have changed so much (maybe not as dramatically in baseball as in gymnastics but still) in small increments over the decades that its basically a different game.
Walter Johnson, who was probably the hardest throwing pitcher who Ruth faced in his career, had a fastball that topped out at about 90mph. Today even guys that never make the majors can throw that hard. And that is just one example. Ruth would be facing a completely different, and much higher, standard of pitchers than what he was used to.
There are a lot of other things too which are worth mentioning. Making it to the majors was undoubtedly a lot easier in his time. The argument made in some comments that MLB quality has been watered down because now there are 30 teams is, I think, completely wrong. The population of the United States in 1920 was 106 million, today it is over three times that. So the number of teams per capita was actually higher back then that it is now and I think that is a more accurate metric for judging that - the number of MLB players is higher now but the number of people competing to become MLB players is also much higher. You also have the fact that in addition to the US population increasing, you also have players from numerous other countries playing in MLB now, so the actual talent pool it draws on is even larger still.
Also a career in MLB was nowhere near as lucrative back then as it is now, so a lot of talented people who could have had HOF careers in the early 20th century may have opted to pursue other careers (Lou Gehrig almost became an engineer instead of a baseball player, perhaps other equal talents made the other choice).
Then of course you have the usual arguments about segregation, the lack of a minor league system to develop talent, differences in sports medicine and training regimens, etc etc. There just really is no comparing the games across that much time.