While a fountain pen may look very straight forward, a "tube" with a nib stuck on the end, but it is actually a more complex invention. The delicacy of gravity-based ink flow, numerous parts all made to precision.
The fountain pen is composed of many parts. All have to fit together perfectly. This is no small feat. As the pen is held in the hand, there is a precise tactile test to meet. When you hold a pen, you will immediately feel is something is not right.
The feel is one of the aspects that you can notice between an expensive and lower-priced pen.
So not only is a fountain pen complex and different because of the multiple parts used in its design... clips, bands, caps, ink feeds, nibs, filling sysems etc., but it is also different from a disposable pen. You will keep your fountain pens for year. Most likely, you will pass it down to a friend of family member.
It also becomes "your" pen. You carry it around with you. It becomes one of your possessions. It is not use a plastic pen that is tossed when no longer needed.
The writing experience is very much defined as to how the pen sits in your hand and how the nib and the ink flows across the paper.
The fountain pen has made quite the evolution. Starting as a quill. The pen then progressed to a pen using steel nibs that were attached to a stick and dipped in ink.
The pen next progressed to a writing instrument that actually held its own ink. With this step the pen became portable.
The first pens had a range of problems. Getting the ink to flow our in a controlled way was one of the primary challenges.
In 1870 Lewis Edson Waterman invented the now famous system ink system to control the flow of ink from the ink chamber to the nib. He developed a three-channel feed that would allow air to travel up into the ink chamber, releasing the vacuum and allowing ink to flow, and not gush out.
Pens went through some innovations. At one time the only way to get the ink into the pen was to use an eyedropper. Then pens progressed so that ink could be drawn up into the pen body through the nib.
Various ink filling mechanisms came along all basically pushing air our of a chamber to create a vacuum to pull the ink into the chamber. Eventually, even a cartridge to hold ink was invented.
What we have today are well engineered fountain pens which, overall, need only a reasonable amount of care to provide a lasting writing experience for the owner.