I wonder this too. I think it's a mix of:

Some people (maybe newer folks) don't realise they can give more claps, or that we earn more the higher the number they give.

Some people remember the old Partner Program rules. I wasn't around at the time, but there are some older articles quoting guidelines that for claps, earnings would be calculated as a percentage of how many claps that user gave everyone in a month. So if someone gave 1 clap to every article they read, each article would earn the same amount as if they gave 50 claps to every article they read. Way more efficient for the reader, but I don't think it works that way now.

And then I think some people are self-promoting by giving a clap. My read stats are still low enough that I can often tell who read my article and who just viewed it, i.e. a 'clap and run'. They clap once, but their avatar and their self-description (often promoting a business) are then visible in your claps list for anyone to see.

I don't always give 50 claps, but I never give fewer than 10. Sometimes I give a random-looking number to make the article's number of claps a round total (i.e. they have 953 claps, I'll give 47 so it totals 1000). That's just me being weird though.