It slows down the iteration cycle - when you make a change to your API definitions you'd need to push the change to either a schema repository or into productio

It slows down the iteration cycle - when you make a change to your API definitions you'd need to push the change to either a schema repository or into production, then regenerate the API - at which point, if you've accidentally introduced a breaking change, your frontend is already broken - you can work around this with CI that detects breaking changes, but it requires a fair amount of work. Having them in a monorepo means that breaking API changes can fail CI and never make it into production.