Why does it do this? And is there a way to change it I'm not finding?
1) Because your ship, in this loadout, can only cover 26 (and a bit) ly per jump. 28.4 max with an empty tank, but the plotter always assumes a full tank (and whatever you got in your cargo hold at the time of plotting).
2) Sure. Add Neutron stars to your selection when plotting the route, and enable jet cone boosting in the route plotting options. Just keep in mind to check on the FSD's health every few jumps and repair it before it falls below 80%. Scooping from a Neutron stars' jet will quadruple your single jump range. Of course, the downside is that the process itself can kill you.... Just don't try to scoop from White Dwarfs - those are pure evil.
Overall, if you plan your trip through one of the four
Neutron fields, you can increase your average speed (in kly/hr) by a factor of roughly 2..3. Not quite the full factor 4 from the increase in jump range, since setting up the jet cone scooping will take a bit longer than regular scooping, and you have to take a break every now and then to refuel from a regular scoopable star (the auto route plotter takes that into account) and to repair the FSD.
The other option to increase the range for a single jump is to use synthesis - FSD boost, also called "Jumponium". That uses up some (more or less hard to come by) materials in exchange for a jump range increase up to a factor of 2 IIRC. Can be useful to make a short cut across a gap in the outer arms, or to permit you to reach a scoopable star with the last dregs of your fuel.
Whichever method you use to boost your range, though - make triple sure in advance that you
can get out from where you jumped to.