Debate question: Is "professional attire" classist?

I had this discussion with my family over the weekend. My mother works for a global IT company, and my sister is a higher up accountant for the IRS (but currently a WFH). They were complaining about buying new business casual attire for upcoming meetings and events. This conversation went along these lines.

"I have to be there for a week, and I have nothing to wear that still fits me. It's been years since I had to buy clothes for thus, now I have to go shopping in the little time I have."

"Why can't you wear your normal clothes?"

"It's dress coded. We're required to dress professionally."

"So you're not professional in your jeans? Do you not submit the same level of work in a tshirt?"

*silent stare*

"It's required. 🤷‍♀️"

"By whom? What will they do if you show up in tennis shoes?"

*silent stare*

"You wouldn't understand, you only report to local government."

"Yeah, and I wear whatever I was wearing before I went to the meetings. If others want to wear suits and ties, good for them. I have to wear work boots, I'm not going to buy heels I would never normally wear, just to present my work to a bunch of white men in suits."

*silent stare*

"You wouldn't understand. You have to keep the same dress code."

"So you're just trying to impress a bunch of white men by wearing pants that don't have pockets. White men that are below your pay grade even."

*changes subject*

What am I not understanding? These women are at the top of their fields. They set the tone for other women there. They wouldn't be fired or seen as less than for not wearing slacks they had to go out any buy specifically for the meeting. Only wealthy people have tailored suits they wear on the regularly. The average person has to spend their hard earned money on specific clothes that they would never normally wear for a single meeting. I retain that "business attire" is unnecessary. You're still the same person, you're not "less professional" because you're in a cotton tshirt.

What do you guys think? Is business attire an outdated and classist form of making the average Joe feel more inferior in a high business environment? Or do you think it's necessary? I'd love to know why if you do.