It's mostly because it's was a different scarce resource at that time that was seen as non-optimal use by the players, the courier. It can ferry item to you, in a normal game there was only one of them for your whole team, which mean using it would take that ability away for your teammate during the ferry time.

One constraint to those showmatches at the time was that every heroes had their own courier, and player at that point were not accustomed to using it for "low value" travel, unlike the AI that was using it liberally.

In a later patch, the 1 courier per hero feature was added, and now pro players are much better at managing it, but at that time it was truly a heavy opportunity cost.