USGS is in the process of collecting that data right now, it's not from the archives, and DEM is different from USGS aerials (which are photographs) and run out of a different USGS office. This is sort of irrelevant.
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.
USGS has been providing free public access to DEM data for ages. The SRTM has been available via FTP since at least 15 years ago when I first started using it to render hillshaded maps. There's not a secret handshake needed.
and the DEM has always been in a native digital format. The whole problem here is that the aerials and conventional maps are not, they're on paper and film and fiche. It takes a lot of time and money to get it digitized and available and USGS was not able to do that for a long time. You could argue that Google's generous offer to digitize the EROS archives contributed to the delays on this.
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.
Really I'm struggling with your statement about digital distribution being new for the USGS. We're talking about an agency that ran a finger service to inform people about recent earthquakes!
I had to order some maps over Antarctica by fax a year ago. The USGS had a functional webshop, but it only served the US, everybody else had to fill out this form including debit card data and send it over. It turned ou my uni actually still had one (1) functional fax machine.
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.