No custom alt text option available for images.

InDesign does not export graphics as an element of their own; all elements must be in a text or graphics frame, which becomes the overall container for all elements — an image, added graphic elements, etc. So it's to that (InDesign) element that alt text is assigned, so that it will apply to the entire exported/viewed object. This is no different from how, say, an image is handled in a web layout, with a formatting block that contains the image link, size and border info, class... and an alt= string. HTML does not "apply the alt string to the image" but bundles into into a single document element.

Which is exactly what ID does as well.

I suggest that (1) you don't know what you are doing understand InDesign's document structure well enough to grasp that a element like alt text has to "go somewhere" just as it does in a web page, and (2) being pointlessly rude isn't going to get you answers and help here very long. Rao's answer, like most given by the Adobe Employees who monitor these forums, was a request for general information that often leads to a useful answer. Most technical answers come from the forum users — this being a user-to-user forum, not an official support one — and we often have to start with just those questions to be able to advise posters who are on different platforms, OS versions, ID versions and such. The employees often jump in on such questions to give a quick initial response and, hopefully, get a followup post of that useful diagnostic info before another user has to do so.

Maybe keep that in mind next time you ask for help?