It's aesthetics/psychological. Weight is also important to me because I travel and the thinness affects my perception of the weight. It's a bit strange that the new Pro feels thicker than my old laptop, the 2015 Macbook Pro, despite actually being thinner. It's because it doesn't have the taper that the old laptop has. Take a look:

Old: https://support.apple.com/library/APPLE/APPLECARE_ALLGEOS/SP...

New: https://awsimages.detik.net.id/community/media/visual/2021/1...