Are you familiar with ICOs? You don't "easily charge USD" because all of them are held using crypto. Yes, some have used bitcoinsuisse to allow some to preorder their slots/amount using fiat, but that's the exception, not the norm.

Moreover charging fiat only prevents them from legally allowing US citizens to participate whereas using the blockchain, they really only have to "warn" US citizens that their participation is not allowed.

People have made a fuss for cashing out (e.g. Status was trying to cash out on a sizable chunk of ETH from their ICO, which led to a lot of angry ICO participants calling them out) in the past. It shows that they don't fundamentally have faith in the system if they're building an ERC20 token but they're selling ETH. No one minds them selling a bit for runway, but if you're liquidating your entire ICO amount, it'd be a huge red flag.

Give me some counterexamples if you'd like to disagree but I'm fairly convinced you're not too familiar with the landscape.