Perhaps because -oh I dunno- in the 18 years since it's release nothing has delivered an overall package that has topped it, including its own sequels?
Perhaps because games made today are made with the shareholders' interests first and foremost, rather than the customers and designer's vision like in the 90s/early 00s?
Perhaps because in general (I know CDPR are an exception to this) modern games are released half baked and fixed never?
Perhaps because the very nature of a Deus Ex style game is incredibly difficult to construct, and developers are more limited than ever in what can and can't be portrayed?
Honestly there are loads of reasons why we generally expect modern releases to pale in comparison to their older, more feature rich ancestors. Star Wars Battlefront, Battlefield V, Serious Sam, Tomb Raider, etc etc.
I certainly hope Cyberpunk can live up to or surpass Deus Ex. But make no mistake, the onus is on Cyberpunk to break the mold of all the other failed attempts at meeting that bar, not on Deus Ex to simply be untouchable or otherwise.