It is a recurrent theme though, one I honestly don't understand. It's like the (probably apocryphal) story of the Army trying save money by having only 1 uniform size. They took the average of all the soldiers and mad the new uniforms to fit that average. The uniforms ended up not fitting anyone.You obviously can challenge players. You just keep raising the difficulty of encounters until you find the breakpoint. Sooner or later you will get dead PCs. That part's not difficult.
I feel that there is a problem here, but the way it's being stated obviously doesn't really get at the roots of it. A lot of people have the experience that 5e isn't particularly hard so there's clearly something behind it, but it's clearly not because it's impossible to challenge the PCs.
Clearly it has something to do with the nature of the challenges on a different axis to whether or not a PC actually risks death. I have some thoughts on this, but I'm not sure it's worth it in a thread not specifically devoted to this topic.
There is no simple formula for encounter design, there are simply too many variables. Are the default encounters targeted to the lower end of difficulty? Wouldn't surprise me at all. After all if you're new to D&D and every encounter is a slaughter-fest, you probably aren't going to come back. The game would not be nearly as successful as it is if you had to be a system expert for your PCs to survive.
I think there should have been more discussion of adjusting the challenge level of encounters in the DMG, but no one set of rules will ever work for everyone.