Most eye doctors use the Snellen chart because it was designed very intentionally. Certain letters help detect astigmatism better, for example the V on the bottom row is often confused for a Y. E can look like B, D looks like O, etc. The font is also very specific and the spacing and thickness of the lines are also scientifically derived. A minor thing is that doctors also have the chart memorized and are observing the patient carefully and seeing if they’re squinting, tilting their head, slowing down, etc. We don’t want to also be checking if you’re right about the letters too, so it’s one less thing to be multitasking on. There are also a couple other intentionally designed charts, such as the ETDRS and the Bailey-Lovie chart, which has a few versions to minimize memorization. But these tend to be actual paper charts (often used in vision screenings or for specialized vision exams such as low vision). Most projector charts are Snellen.