The problem is that stuff like privacy and open source don't attract the vast sums of venture capital that ad-selling businesses do. Sure, an average dev could probably build a working app like that - but could you ship the same quality as someone who can afford to hire an entire UX team?
Probably… it would not make you sit through 30 seconds of ads so that would be a great UX improvement.
Never mind quality - how can you achieve the same reach as someone with an advertising budget?
No, but people don't care about UX perfection in comparison. Usually it just needs to work and not get in the way.
imo, the real problem is that you need to consistently ship. It you made a one-off app then it's going to get de-listed for not being updated despite not needing an update.