Why does Bitcoin Core not support showing the prices in USD or other fiat currencies?
Why do they not want to add this feature?

There is no &quot;they&quot;. You can create issues or open pull requests to see feedback about a feature request from others.
Few reasons I would not want such feature in a bitcoin wallet:

Bitcoin Core wallet is an open source bitcoin wallet which is used by people from different countries with their fiat currencies. Lot of currencies and API for exchanges will be used to add such feature.

Bitcoin wallet doesn't need amounts mentioned in other currencies when it has its own currency which is BTC. I do not want to see amounts mentioned in EUR when I login to my bank account in India. Amounts mentioned in INR works better.