I can't speak for others but HSTS is a major reason. Not everybody wants to deal with setting up certs for every single application on a network but they want HSTS preload externally. I get why for AWS the solution of having everything from a .com works. But for a lot of small businesses it's just more than they want to deal with.
Another reason is information leakage. Having DNS records leak could actually provide potential information on things you'd rather not have public. Devs can be remarkably insensitive to the fact they are leaking information through things like domains.
Another reason is information leakage. Having DNS records leak could actually provide potential information on things you'd rather not have public. Devs can be remarkably insensitive to the fact they are leaking information through things like domains.