Tooling is expensive (The tools used to mold/cut/grind the chasis of the device).

It would take them many iteration to perfect the tools needed to create a brand new chasis. Higher iteration, higher RnD cost. High RnD cost = High price tag.

It makes a lot of sense for them not to reinvent the wheel again if they want to price their product lower. It's very smart of them to leverage their previous designs including the logic boards to create a brand new product.

Even though they are recycling the old designs, I'm pretty confident the quality is just as high as the ones before. The rigidity of the chasis, the premium feel of the metal/aluminium frame remains in tact. I doubt any other OEMs can replicate what Apple did here.