Did he though? Giggs was shagging anything that moved, including his sister in law. There was a period in the 00s where Becks was known just as much for his "celebrity" status as his football ability. Keane spent a night in the cells towards the end of the Treble season.
Ultimately, they performed well on the pitch. Fergie kept them around because of that. The second that performances started to drop, they were binned. Giggs stayed because again, he always maintained consistency on the pitch.
There was also a stability around the club which also made it easier for these lads to perform even if they had things going on.
Now, in the current team, many players are making mistakes. The difference is now, the occasional indiscretion (anything from a cryptic social media post to getting pissed in the week) is deemed a massive crisis. There's a dozen back page stories about it and a million social media posts. The unwanted media attention then becomes a distraction and eventually becomes a contributing factor to poor performances.
Speaking to journalists from the 90s, they had all sorts of shit on some of our players but Fergie had a politician esque way of dealing with the media. As a result, stories never get published. No distractions. Positivity in the camp. Granted, this is harder with social media but even now, there's generally a few dozen people that control the narratives around players. If you get them onside, the narrative changes.
As an example, If Foden was at United and was "allegedly" doing the same things off the pitch that he was doing at City, there would be stories every day about him that dominate the news cycle and trending every day.