There's no overwhelming financial incentive in open-source applications, while both mobile stores are a constant arms race in monetization, advertisement and shady practices, where legitimate and privacy respecting alternatives are relegated to obscurity and difficult to find even if you're specifically looking for them by name. In some ways FOSS software is even discouraged from being published due to some rules (see for example donation links being strictly forbidden) and the probability of fake malicious apps, like NewPipe or SimpleMobileTools clones.