Maybe because people haven't actually watched him play, nor do they know what Jose is planning and what type of player he needs. Basically the logic is Fabinho is a DM, he played well last season for a team who played very well. Therefore we should sign him and he must be amazing. It's incredibly simplistic and we have no idea whether he's the right fit for Jose.
I'm not against questioning signings and debating the merits of these things. I'm just sure almost no one has watched Fabinho play more than once or twice and actually has first hand knowledge of what type of player he is.
For what it's worth, I think there's no guarantee this guy comes in and is instantly world class, he's played 1 good season at DM and we don't know how he will fit in our side and whether his qualities are the ones we need. In my mind the perfect DM for us is a Kante type, very energetic and covers a lot and protects the defence. I've seen Fabinho described (from people who've watched him) as a Tiago/Gabi style from Atletico and from that it doesn't really seem like he's the Kante style