Save it as an AI file instead of EPS. I just did a test with a placed JPEG and an AI file.
I used the Object Export Options dialog and set a custom Alt tag for each of the graphics. I exported as PDF (Print) and made sure I turned on Tagged Text.
Both graphics had Alt text and passed the Full Accessibility Check in Acrobat Pro.
I have done all this and it still doesn't work. I have set up presets for a client who requires all the accessibility settings, so have included 'create tagged pdf' etc.
I'm on a mac platform. I wonder if there is a problem with the mac version?
Very strange. I have been doing all of that and the alt text still doesn't show when I roll over the image but when I look into the tag properties it is there. So it passes the accessibility check but the alt text doesn't show.
When I create a button in an interactive PDF and I add a description, that is a "tool tip". It has a dual purpose, as I understand it: It popups up when I roll over an image, and it also acts as Alt text for a screen reader.
However, I don't think Alt text is expected to popup when you rollover an image. I think it is only read by a screen reader.
You might ask more in the Standards and Accessibility Acrobat forum: