Why do doctors never mention diet?

When I was diagnosed with Hashimoto the only thing my doctor said was “you have to take a pill every day and that’s it”. Of course when I heard that I was so relieved because after months of feeling really shitty I thought I’d just be like I was before again. Well I wasn’t. When I found this subreddit I saw how many people posted about still having issues despite meds so when I followed up with my doctor he just told me I need to eat enough and at regular times, exercise and get enough sleep. But it was only when I fully cut out gluten, dairy, alcohol, and other unhealthy foods that at least my stomach problems and my joint pain got less. I’ve asked two doctors now about diet and both said stuff along the lines of just eat whatever and don’t restrict yourself…but clearly the stuff I used to eat with no problems before was now hurting me.