Because clicks are free to report and calls are work to report. Every ad platform hands over impressions, clicks and cost per click for nothing. A report built from what the platforms hand over will always be a report about clicks. Calls have to be tracked, recorded, matched to their source and reconciled against the CRM, and each of those steps is something the agency has to have built.
It is not usually dishonest. It is usually the path of least resistance. But the effect on you is the same: you are shown a number that went up, and the number you care about is not on the page.
What a click report cannot tell you
It cannot tell you which channel the job came from. It cannot tell you whether a call was a customer or a wrong number. It cannot tell you that the form stopped delivering three weeks ago, because the form still shows a thank-you page and the click still counted. A click report can be excellent for a month in which you booked nothing.
What a call report has to include
The call, with the recording, with the source it came from, and whether it was answered. Every form submission, from the raw log, not from the CRM's count of them. The spend, next to the calls it produced. And where two of those disagree, which one is believed and why.
A report that includes all of that is a report about your business. A report that includes impressions and a ranking checked from one spot is a report about the platforms.
What to ask for
Ask for last month's call log with the source on each row. Not a total, the rows. Then ask how many forms the mail server sent and how many the CRM shows. If the agency can produce both without a delay, the tracking exists and the report just needs rebuilding around it. If it cannot, then no one has been watching the joint where your leads go missing, and the click number has been standing in for the truth.
The short version
A report should start with calls because calls are what you sell from. Clicks are how the calls got there, and they belong further down the page. If your report is the other way up, ask for the rows. You will know from the answer.