When these fake name/number sites first appeared about 5-8 years ago, they didn't have any ads or even outgoing links. (I looked without an ad blocker.) Ads might be the explanation for many of them today, but I don't think it's the original or sole reason. Though I suppose it may have begun as a proof of concept without ads to gauge the traffic, then ads got added years later.

I've read speculation that they were started by telephone spammers to poison the utility of those who-called-me websites that highlighted spammers' phone numbers. I don't think that's the real reason either. It sounds too cute and clever for spammers. (As an aside: nowadays those services are useless since spammers can so easily fake the caller-ID.)

I'm still thinking that there is an interesting story for the original purpose.