This is... not completely true.
I have a semi-retired StJ/SR Stalker (an AT that shares the same defense scaling as Scrappers) who I've run through Incarnate content for a while, and got to Vet Level 20ish. I built her heavily with survivability in mind, under the premise that a StJ Stalker doesn't really need a ton of help offensively. When I had her fully built out for durability, she was defense softcapped to Incarnate content, and sat at 51% S/L resist at max HP. By half health, that's already up to resistance levels you're talking about. Now granted, I had to do a lot of sketchy things to get her there, like prioritizing Mako's Bite over purple sets, and I rebuilt her a little more "responsibly" later on - with the rebuild, she was somewhere around 43.5% S/L. That's still hitting those resists at around 40% health. It IS possible to get fairly decent resists out of /SR, but you have to work for it.
@Werner was a little more on the money - the big problem I've found with /SR is the lack of sustain, ie, healing (the other problem for me was lack of HP, but that's much more a Stalker problem than it is a Scrapper problem). You can take external steps to fix that, but as always you have to make sacrifices to get there. For my Stalker, it was too many pressure points for soloing high level content, but on iTrial teams, she was extremely durable (as well as still outputting stupid damage). As you move through the more durable ATs, where the extra HP helps /SR disproportionately, I think you'll find /SR starts losing a lot of that stress I found it under.