Because lost in all of this excitement to tank, tank tank...there's also players on your current roster, and/or who will be integrating the roster in the coming years, and they need to be surrounded properly as well.
This is as much part of the "rebuild" as just tanking and drafting 1st overall.
You know...so we don't have Oilers-like situations where they've got Connor McDavid centering Zack Kassian and Drake Caggiula because they never cared about anything else then tanking.
We've been rebuilding before this year, you're just fooled by the Cup run and thinking this is going to start this year.
It already started, we've just got someone more capable of handling that process now
Most often that not, "Rebuilding" a team means taking a group of veteran, but (mostly) mediocre players, and trading them for younger, unproven players, or good picks, in the hope that the younger players will collectively "grow" into something better than what you started out with.
When was the last time the Habs seriously did this? Actually, they lost their young high pick in Kotkaniemi which was a pillar in the start of a start of a rebuild attempt around 2018 (which actually turned quick in a rushed retool) when we traded Pac and Plekanec, because they refused to pay him/ failed to properly develop him and rushed him instead, and traded valuable picks as a knee-jerk reaction to get Dvorak. They haven't moved any older important pieces of the core to aquire interesting futur assets beside Shaw three years ago (unless you think moves such as Kovalchuk for a 3rd round was part of a great rebuild masterplan). They haven't been any true effort to really rebuild the core of this team in the past 3 years, Bergevin betted on his vets like Weber, Price, Petry and Gallagher instead of trading them while there value was still high.
Only thing they did is rush the rebuild by putting a 18 years old rookie on the team instead of being patient, making some "hockey trade" like Anderson for Domi, traded a couple of picks for Edmunson, Armia, Cousins and Allen, signed Toffoli, Bergevin tried to do some quick patch work instead of properly give a chance to the Habs to build strong foundation for years to come.