Why is there no decentralized Bitcoin trading API?
I'd say that's just because nobody programmed it. Bisq community for example is short on programmers so they concentrate on the essentials. Companies have no interest in the development or shy away from the legal risk, where they are increasingly forced to perform financial police duties similar like banks (KYC/AML). Hopefully the Bisq DAO brings enough funds to have developers paid for this kind of work. 
Another concern is security. Successful replay or DOS attacks or any essential coding error might risk the whole Bisq project. Like with Bitcoin it's not easy to force the users to upgrade to latest version in case of malicious code.

Do you know the latest developments at the Bisq API? Have a look at this developer conference. There is as well a slack channel for the Bisq API interested.

Record of Bisq developers API discussion, Sept 2019