Keep in mind that when we talk about the EROS archives we're talking about data that goes back to the 1930s and earlier for some product types.
For a long time I got the topo maps from the website of a state government bureau that had conveniently run them through their own large-format scanner and posted the TIFFs - USGS didn't get around to it for years after. It's hard to blame them too much as they had a shoestring budget.
Actually, for amusement value, that state agency appears to have removed the TIFFs from their website and now says that you can order the topo maps by mail for $8 a piece, which is what I used to have to do. I wonder if USGS got mad at them, which is a bit ironic since they don't mention that USGS themselves only recently started offering them online for free. For additional amusement value EarthExplorer, the fairly new service that lets you retrieve aerials online, has a banner up that downloads are intermittently broken and indeed I can't get it to work at the moment.
Making digital data publicly available is pretty new for USGS. Just a few years ago archived aerial imagery had to be ordered by mail and it was a pretty lengthy process. Topo maps (the earlier equivalent of the DEM data to which you refer) were generally ordered on paper as well up to five or so years ago, but they're in a lot more popular use so more third parties got into the business of distributing them. I've relied moderately heavily on both for some of my research and was a very painful process until just recently to get anything older than current. In the meantime, yes, Google had it all at some point, but mostly stopped using it or providing it because they obtained better quality imagery.
Fortunately USGS now has a slippy map for topo and an admittedly rather clunky ESRI query service for aerials.