I think because of history when payments were clap related, and because of cognitive biases.

Here are some potential ideas.

We see the number of claps indicator and think it’s important just because it exists. (while reading time is the only one important as you said, but less visible, you need to check the stats details to see it)

We’re “sad,” because reading time is low, so we look for ways of compensating and find some satisfaction in high numbers of claps.

I would be interested in reading the results of a poll titled “Why do you clap 50 times!” :D