Ad algorithms are great at finding conversions. They're terrible at finding your best customers.
They can't tell a one-time buyer from a repeat customer, or an existing customer from a new one. So they burn your budget acquiring low-value shoppers and waste impressions on people who have already bought from you.
Ad algorithms are great at finding form fills.
They're terrible at finding revenue.
They don't know which leads will become paying clients and which go nowhere, so they bring you more and more junk leads. CPL looks fine, but cost per qualified lead or closed deal is far to high.
Ad algorithms are great at finding leads.
They're terrible at finding pipeline.
They don't understand which conversions will become qualified opportunities, so they optimize as if every conversion is equally valuable. Lead volume and CPL might look good enough, but pipeline stays flat.