They do have a tendency to get killed, and it flabberghasts me to see how often they get killed despite the fact that they don't have to. Solo adventurers sometimes do silly things like wander off alone out of pique and not out of necessity, fail to coordinate with each other, stand and fight when they ought to run away and hide, fail to sneak, or push big red buttons marked "If You Push This Button You Will Die".
When I refer to reconnaissance as smart and effective play, please understand that I'm pointing out the benefits of doing it well, not poorly. (Sometimes I explain the methods of doing it well. This doesn't imply that my players all use these methods all the time. Some do.) You're right that for an unskilled player, sticking with the group is the path of maximum safety; and you're right that a DM can often get away with providing easily-curbstomped challenges and yet the players will still have a fun and rewarding time failing to curbstomp them. That's the reality of 5E as a casual game.
Ive been DMing for over 30 years. Ive dealt with all kinds of parties, both competent and incompetent across four editions of Dungeons & Dragons (I skipped 4E) and BECMI, Role Master, WEG D6 system, Shadowrun, Spacemaster, Earth Dawn, Middle earth role-playing, Pathfinder, Saga edition, Rifts, Cyberpunk Dragon warriors and other role-playing games. I've learnt a lot in that time. I've encountered similar problems to what you and captain zap have encountered. Ive DMd for hundreds if not thousands of different players.
I can tell you that in every single one of the times that happened it was the Dungeon Masters (my) fault. It is almost never the players fault. It is almost never the systems fault. I've learnt that players will conform to the path of least resistance. If your players are defaulting to similar builds and tactics it is because the game as you run it, channels them in that direction.
And most of the time as DM you won't even realise you're doing it.
The classic example is with 5Es encounter system. I lost track of the amount of dungeon Masters complaining online that they are too weak, and are getting easily curb stomped by nova strikes. The solution they propose is invariably always to numerically make the encounters more deadly. Of course this just channels those same players into Nova striking simply in order to stay alive. Suddenly builds that favour nova strike tactics are the norm.
Rather than solve a problem, all this dungeon master has done is enshrine it.
As dungeon master if you want the monsters attacking the player characters at close range, just do it. You don't need a raft of complex mechanics in order to do so. When you sit down to design your 6 to 8 encounters for that adventuring day, Have one of the encounters feature a mage with wind wall. Have another encounter set up with the monster simply teleporting in next to the players, or bursting out of secret doors with a DC equal to the passive perceptions of the players plus one. Or concealed via illusions as friendly creatures. Or in an ambush of their own. Intentionally contrive encounters the trap that scouting shadow monk, and have your players thinking outside the square.
I'm not saying do it all the time. And I'm not saying be a jerk about it. But nothing is more boring than doing the same thing over and over again. Catch the players offguard. Force them to think at a rapid pace. Make them make decisions under pressure. Have tactics that work one day not work the next. Remember you have the discretion as dungeon Master to design your encounters as you feel fit to challenge your party.
And of course to set up some encounters where the PCs tactics are optimal, they catch the monsters with their pants down and curb stomp them. If your wizard has just got access to fly let him showing off by decimating some creatures from the air. Then set up an encounter designed to knock him out of the air. Get him thinking about the spell as a risk versus reward thing.
If you're seeing the same thing happen at your table over and over again, stop and consider that you might actually be the problem. I'm not saying that to be a jerk either Or to say that youre are bad DM; none of us are perfect.
I just can't help but shake the feeling many of the problems you and captain zapp experience are down to your DMing style and how the players react to it.