Except, actually it's literally not? ^^;; Ink, that is. Definitionally. Toner (in consumer available laser printers) is a fine (dry) plastic powder (that then gets electrostatically attracted and melted onto the substrate).
Total pedantry aside (sorrrry), when it comes to technical uses, even ink really isn't just readily interchangeable. Even something "simple" like with fountain pens, you'll see issues with different inks. Various screen printing inks (where the viscosity wouldn't work in many other things you'd expect to be using ink with)... plastisol is really quite different from both acrylic (paint) and water based printing ink, and then in turn from discharge ink (really a heat activated chemical dye remover). Then it comes to inks in something as technical and specific as various digital print heads. Then things like DTF/DTG inks. UV inks. Eco solvent/solvent dye inks. Dye sublimation inks. Various varnishes you can run as ink. There are inks you can run through a reasonably appropriate printhead that will clog if you don't print daily, and will eventually clog either way.
Even "Gutenberg's ink" wasn't... "just ink". It was an invention using oil based ink (based on contemporary progress with oil based paint) that had better wetting and transfer properties and achieved dark black prints, as opposed to the more commonly used water based inks of the time that were poorly suited to letterpress.
I mean there's an expectation that with third party ink or toner, they'll "get it right" for the appropriate printers. But... "getting it right" involves having the correct ratios of solids milled to the correct fineness, or the correct dye chemistry that will stay consistent and not fuck up the print head (especially if reacting with previous ink used), and the correct liquid medium chemistry to carry it, in the case of ink. Different inks dry faster, perform differently with multiple colors, etc, and it quickly affects print quality even when it runs through the printhead just fine, some of which can be compensated for with slower printing, different patterns, etc. I use third party inks, but also... ink isn't just ink.