It probably isn't wildly lower today, we know of at least five or six big supernovae in the Milky Way in the past millennium. For 200B stars in our galaxy the size normalized rate implied by that would be like one ever 300 years. So if you extrapolated the Milky Way alone in (cosmological) modernity you would get 10/sec not 30/sec.
There is dust between us and most stars in the Milky Way that blocks them from view in visible light. Therefore we can only see a fraction of the supernovae in the Milky Way.
It is substantially easier for us to see supernovae in other galaxies that we're not facing edge-on. And we have a large sample size of such galaxies. That's why our best estimates of supernovae frequency are based on observations of such galaxies, and not on our observations of the Milky Way.