Their games aren't as exploitable cause their base is already very strong, while having the physical aspect aswell. They are incredible movers for their heights.
Their bases aren't all that strong. Competent, yes, but not strong. At least not strong enough to cover up the holes in their games.
With Medvedev, his whole game is the problem: he doesn't have a consistent weapon so he gets pushed to the defensive (which he does handle well) way more often than not. Without a truly elite weapon, Med falls into the Murray category of not having a game that's remotely threatening for any top-tier ATG (and I'm not even talking just the Big 3 here), except Med just does most things even worse. He works with what he's got because he's easily the best tactician from the next gen and his shots aren't outright bad, but it's clear that he's working with some limitations. You can see this in his forehand.
Zverev's forehand is also a completely fine shot, but it's still too passive. There have been countless instances when he could have used it to shift the point in his favor but he instead uses it as a lame rally shot most of the time. It's the same problem Murray's forehand has, though I do think Murray's might have been even worse in that regard; that shot was a genuine weakness in his game rather than a "not-a-strength" as it is for Zverev. Zed's got a backhand and (first) serve though. It's a little bit puzzling how he hasn't done better with the strengths he has which suggests that I might need to place more blame on his mental strength.
Tsitsipas's forehand is actually a good shot, I like it. His problem is the backhand which is a bit loose and prone to shanking. Subjectively, it's kind of ugly for a one-hander imo. It doesn't do anything at all for him, an issue that could be resolved if he decided to slice it more: I rarely see him employ the slice. But the real weakness in his game is the return: it's an awful, awful shot for a top-tier player. Clay hides that weakness a bit (which is a big part of why that's his best surface) but it's a liability on hard courts and an instant KO on grass. I do think his fitness is also decidedly mediocre (and it showed in the RG 2021 final).
Thiem has the best "basics" of all of the next gen (but he's not one himself). Great forehand (and I mean an actual ATG forehand), pretty good backhand, and a decent serve. He's a technically sound player, which can't really be said for his younger peers. One hole in his game is the return which isn't that great, but with Thiem the problem is mental: most of the time, he goes completely AWOL in the big finals, and not just against the Big 3 either. We all know how the US Open 2020 final against Zverev went down, and I wouldn't call his ATP Finals 2019/2020 losses much better in that department. It's frustrating as someone who actually really enjoys watching Thiem because he has a great game but his mental strength is so lacking that all of those advantages disappear in Slam finals (or even Masters finals lmao).
Whether it's physical or mental, all of these players have noticeable weaknesses in their games, and their strengths aren't good enough to counteract them (due either to them not having such effective strengths like Medvedev or having such massive weaknesses that they'd need
really good strengths to make up for them like Thiem). Now let's look at Roddick, the person everyone loves to poke fun at for having holes in his game.
Roddick has a pretty laughable backhand, I'll be honest. And his return wasn't great either (barring a few select matches like the Wimbledon 2004 final which featured pretty stellar returning from him). But he actually had elite strengths. His serve was top 5 or top 10 all time (lean the former) and his forehand (
specifically the 2003-2004 shot) was an ATG groundstroke. Because both his serve and forehand were that good, his 1-2 punch was devastating. The criticisms levied at post-2004 Roddick are mostly justified because he decided to nerf his forehand into a rally shot rather than a weapon (even if his serve was still an incredible shot), but I won't hear any of that crap about 2003-2004 Roddick. I'd say he covered his weaknesses pretty nicely.