Sorry, in my experience as an interviewer, its generally a choice. You are totally misconstruing the OPs comments..
Parents should bring kids up to dress appropriately, i.e. funerals, evets, weddings, interviews etc. If they miss it then it gets taught in careers at school. Then there are numerous examples in social media and TV. If you really want something, you do your research and you set your stall out.
I have had candidates coming to interviews in thread bear trousers, very clean but old shirt and tie and mismatched jacket, sometimes wearing Dads or Mums suit from a different era. They make an effort, it shows they are interested in the job. On the other hand if I am interviewing for something more practical a handyman, tradesperson, etc, I would still expect to see clean and smart probably not office wear. But trackies are casual wear. You could possibly get away with smart trainers with office clothing but you need to show you want to make a good impression and you are keen to impress. That doesn't mean a designer suit, it just means making an effort.
Turning up for a reception job in trackies and a trainers would tell me they were not the right fit for the role and that they weren't really interested and its just a tick box exercise, unless I knew for example, I had invited them at last minute and said it was OK to come as they were.
The OP passed comment on the trackies and trainers as inappropriate wear for this reception role, she had not written to the potential employer about this, she has written to them about the blatant disrespect and disorganisation of their interview process. It says a lot about the employer that is not complimentary and they need to know they maybe losing quality candidates because of it.