I think what makes Tsitsipas and Zverev interesting is that they're in a weird category of player that is better than the standard 2nd tier types (Berdych, Tsonga, even Ferrer) but don't seem to be able to get over the hump and win a Slam (yet).
I've pointed out that Zverev arguably has the best record of any non-Slam winner, and if he wins a single Slam, he'll also be among the best single-Slam winners (right now I'd say that title belongs to either Thomas Muster, Michael Chang, or Daniil Medvedev - or possibly Tony Roche, if we count pre-Open Era records). He's the only player with more than 7+ big titles and no Slams - and he's won 8, including 2 Tour Finals, 1 Olympics, and 5 Masters. The next guy is Marcelo Rios with 6, and then a bunch of guys with fewer:
8: Zverev
6: Rios
4: Okker, A Medvedev, Mecir, Davydenko, and Tsitsipas.
3: Gottfried, Solomon, Clerc, Chesnokov, Corretja, Enqvist, Nalbandian
2: 17 players (incl. Tsonga)
1: 45 players (incl. Berdych, Ferrer)
So Zverev stands alone. Rios had a short career and a shorter peak, with one of the best Slam-less years in 1998 (Sascha's 2021 might be the greatest Slamless year).
Tsitsipas will almost certainly win at least two or three more big titles, so if he doesn't win a Slam, he and Zverev will stand in a unique position in tennis history.
Or to put it another way, there are 25 players with 8 or more big titles, and all but Zverev have won Slams. And Tsitsipas has already won more big titles than guys like Jan Kodes, Yevgeny Kafelnikov, Manuel Orantes, Petr Korda, Richard Krajicek, and Andres Gomez (all with 3, and 1-3 Slams each).