Porto and Juventus are two different things entirely. We consider ourselves elite, they know they are a big club but not elite. With that mentality they can manufacture and sell to the elite, whereas we see it as a sleight on the club if a player asks to leave to a 'bigger club'.
The other players we sell are either unwanted or we have given concessions on a personal level. Maybe that's not a good way to do business on a financial level but on a personal level you can either hold players ransom, coerce them (and invariably get less out of them), or have them wasting away with a fat salary.
If we were Porto we would have sold Pogba already to the highest bidder, sold Vidal 3 years ago, be selling Dybala in a couple of seasons, would have sold Bonucci. Constantly rotating roster of players because that is what the club is there to do. Be competitive in Serie A? Probably, but winning is different, look what has happened with an unsettled team. It's not Liga NOS where you play nobodies for 2/3 of your fixtures. Competitive in the CL? Definitely not.