Jumps indeed are limited in distance. Near Trantor, they are extremely limited.
The stars were as thick as weeds in an unkempt field, and for the first time, Lathan Devers found the figures to the right of the decimal point of prime importance in calculating the cuts through the hyper-regions. There was a claustrophobic sensation about the necessity for leaps of not more than a light-year.
— Foundation and Empire
The first part of any journey out from Trantor to the outer star systems, and the last part of any return journey, involves passing the Galactic Core region, where hyperspace travel is slow. It's slow because it involves lots of short jumps, and because, until the invention of the Lens as described in Second Foundation, each of those jumps involves a lot of lengthy work.
It was because of that, that the Lens had performed a near-revolution in interstellar travel. In the younger days of interstellar travel, the calculation of each Jump through hyperspace meant any amount of work from a day to a week — and the larger portion of such work was the more or less precise calculation of "Ship's Position" on the Galactic scale of reference.
— Second Foundation
Advances like this happen over the course of the series. You will for example find, as a plot point, an advance in hyperspace jumping that enabled one military force to defeat another in battle. The changes in technology as the Empire falls and through the Interregnum, and an Empire where the capital world is physically remote from and difficult to reach from the periphery, are some of the plot drivers in Asimov's famous "human-only Galactic Empire" setting.
So the answer is: because constructing the setting that way drives the plot.