Instead, they consciously choose themselves to be a monopoly.
Exclusivity contracts give way too much power to Google at the expense of everyone else, if we could simply leave it to Google to decide which websites they want to place advertisements on.
It's like having a retail electronics store placing bad products from company B next to good products from company A. We don't really let A bully the store into exclusivity either, if A doesn't want to be associated with the store they can simply end their contract.
Rather, they are specifically locking everyone on their platform.
I do not completely understand your "de facto" vs "de jure" distinction. Unless of course you don't see any other ways of dealing with your clients but a binary yes/no.