There are basically two compelling reasons to use a privacy coin:
- You are doing something illegal and don't want to get caught.
- Powerful actors in the economy are abusing their power, and you want to avoid/prevent that abuse.
Most people are not criminals, so let's focus on the second use-case.
If the government created a 1000% tax on all person-to-person transactions, you can bet nearly everyone would switch off of Venmo or Cash App and onto something like Monero. Or if Apple/Google said "we are now going to monitor your bank accounts, and we will be taking a share of all purchases made from your phone," you can bet people would switch.
Another type of abuse might be political. "We will be compiling a list of everyone who donates to the opposing party."
One might argue that those abuses of power already exist, and are already actively harming the average consumer. For example, Experian's multiple data breaches[0]. But the thing is, the average person doesn't feel very affected. If you came after their Venmo or their bank account, they'd feel it.
In a way, I think privacy coins are already benefiting all of us. The "powers that be" know that it's possible for us to switch, and that we will switch if they push us too hard. So they don't push us as hard as they could.
I also want to point out: MobileCoin. Your premise seems to be that privacy-focused coins aren't in wide use, but I don't think that's a certainty. Signal has deployed MobileCoin to potentially hundreds of millions of people. But they are secretive about their numbers, so we don't know how widely it's really used. I've tried MobileCoin with friends who use Signal and it works great. They will surely beat out Monero and others, simply due to the ease-of-use they've created, and the integration with an already-widely-used private messenger.
[0] https://en.wikipedia.org/wiki/Experian#2015_data_breach