They may be just didn't find a way to make ads better with payments APIs but seriously, I think the only reason why google didn't get into payments is the customer service burden also, why would the company which depends on ADs wouldn't want to see the result(purchases). Or may be they do have a product but no plan like always.
Don't underestimate companies ability to do 'only one thing' and 'screw everything else up'.
Shopify has evolved on top of 'a neat thing done well early on' and everything is about scaling that.
I've worked at very successful large tech companies who were actually terrible at doing any other tech than the product they sold.
Why isn't Google doing a Stripe API? Why isn't MS? Why isn't Apple? Why isn't Volkswagen?
They are really separate businesses.