It is because currently, Google Ads attribution modeling is only available for clicks on Search Network and Shopping ads on google.com.

It isn’t available for clicks delivered by Search Partners. It is not available for interactions with display or video ads either.

For this reason, Display ads clicks can be attributed in Google Ads only in the last click model.

Therefore:

The Solution

A solution to this problem may be running search and display and in separate Google Ads accounts with a separate conversion tracking on the account level.

(Don’t use cross-account conversion tracking, because the purpose of splitting the accounts is that each account has its own independent conversion tracking).