this is a common intuition-based fallacy. i'm an expert with almost 20 years of experience working in this field with various phd's in political science, math, and game theory. i explain this here:
https://medium.com/election-science/expressiveness-6ef8c034b...
tl;dr - for any two candidates, X and Y, the voters who approve only one of them express a "ranked preference" between them. and because those voters are, on average, a good statistical sample of the X-vs-Y preference of the entire electorate, approval voting turns out to measure voter preferences very accurately.
whereas instant runoff voting can actually have much bigger problems in preference detection because it only looks at one layer of preferences at a time. for example:
35% left center
33% right center
32% center
in this example, center is preferred to both the left and the right by a LANDSLIDE 2/3rds majority, yet is the first candidate eliminated. this happens because of "vote splitting" (spoiler effect) between the two more partisan candidates. so it might _feel_ like you expressed your preferences better. but you didn't really.
and what about STRATEGIC VOTING?
if right wins, then the left supporters will wish they had strategically voted for center to prevent that. and if left wins, then right supporters will wish they had strategically voted for center to prevent that.
approval voting satisfies the "favorite betrayal criterion", which means it cannot possibly hurt you to support your sincere favorite candidate.
for these and many other reasons, approval voting is vastly superior to ranking.
https://electionscience.org/library/approval-voting-versus-i...