Its important to note the time it was made in. 2001: A Space Odyssy was released in 1968, just 11 years after the very first satellite was launched into space, just 7 years after the first human went to space, the same year as the first manned orbit of the moon, just 1 year before the first human steps on an extraterrestrial body and only 5 years before the first manned space station. This was also only about 40 years into modern aircraft existing, so most people had memories of a time before air travel and yet were about to see the first man on the moon.

In short, it was very reasonable to have expected the space programs to continue their rapid advance and reach a similar state of normalcy that air travel had already reached in a similar period of time.

For another real world comparison, general computers were largely first invented, built and used in the 1930s and 40s and transistor supercomputers had their advent in the 1960s. Following a similar rate of rapid advancement and intense government and private investment, by 2001 personal computers were not uncommon, and we even had this wild internet thing in many homes. Imagining computer advances petering out like space investment did would mean we'd still be handing punchcards to university computer operators in 2001 and individual office computers starting to make financial and business sense today