Outside the realm of every day software used by the masses and games, most software I've used in the past has come with 5-6 installation DVDs with 9 GB of data on each disc, expanding to a total of nearly 90 GB on a drive. It just isn't economical to download 54 GB of data in a sitting and that covers only one software, not to mention the GBs its plugins take, each one taking nearly 4 GB alone. Even with a 100 Mbit down connection with a theoretical 500 GB cap each month, I easily get close to that cap doing everyday things. I mean, I really like YouTube, but that aside getting rid of the optical drive pisses prosumers off. I find it silly to buy a laptop and then to purchase a USB DVD or Bluray drive.
That aside, downloading may be fine if your connection speed can support the burst habit, but then there's caps. Most American ISPs (let alone outside of the USA) have silly caps. Most of these thinner profile computers use SSD. Easy to fill those up with a few large downloads, then you need to worry about expansion during setup. While I've heard the line from the average consumer, some of us still rely on discs because the software simply doesn't come on download or isn't feasible to download, as the provider's server speed is atrocious.