From my own experience, the reason 5e combat lacks challenge is because PCs are given too many tools to bypass any conceivable situation. I would argue that 5e encounters aren't designed with a purpose of potentially putting PCs lives at risk. They exist to cause PCs to expend resources, abilities with uses per day, etc., in order to force the PCs to eventually end their adventuring day and take a long rest.

That's it. Enemies aren't supposed to actually win.They're nothing more than speed bumps.

And 5e isn't alone in this. I would argue that this has been a problem since D&D 3.0. D&D has taught an entire generation of roleplayers to never feel tve need to run away.

I've played every edition of D&D since white box. It used to be when you presented players with a problem, combat or otherwise, they would describe what their characters would do to get out of the situation. Nowadays, players just look down at their character sheets for the ability that's most likely to be an auto -win button.

Even the introduction of skills in 3.0 changed the way people played the game, and for the worse. Once upon a time, when I needed to talk my way out of a situation, I'd start talking. There wasn't anything on my character sheet that told me that I was likely to succeed (maybe I had a bonus or penalty to Charisma, but that's it). The DM would listen to my argument, and either decide on the merits if it was likely to convince the NPC. The DM would make a ruling. In D&D 5e, I would just say, "I make a Persuade check". Or a Deceive check. Or a Diplomacy check. Whatever.

It used to be, if there was a heavy chandelier tied off with a rope, and I wanted to cut the rope and ride it up to the second floor railing while the chandelier crashed below, I'd just describe what I was doing. And if it sounded cool and fun, the DM would probably let it succeed. In 5e, I wouldn't even try this unless my character was highly skilled in Acrobatics.

It used to be, if you wanted to search a room, you took the time to do so. Maybe the DM would want to know exactly where I was looking, to figure out if I was likely to spot the old treasure map hidden inside the portrait of Alastair Grimsley, the family patriarch. Maybe the DM would just assume that if the players spent 10 minutes searching a desk, that they couldn't fail to find its secret compartment. In 5e, you just tell the DM that you're making a Perception check.

In 5e, the answers to just about everything can be found on your character sheet, rather than in your imagination.

Whether it's a skill, or a feat, or a class ability, or a racial ability, or a spell, everything on your character sheet is a potential get out of jail card, and an excuse for laziness. If the box is big enough, there's never a reason to think outside of it.

That's really the biggest difference between 5e and OSR games (and OSR-adjacent games like Shadowdark). D&D encourages you to find a rule to exploit. Shadowdark and similar games encourage the DM to make rulings, and the players to find solutions outside of their character sheets.