Aside from the issue of platform owners (Apple, Google, Microsoft) offering storage sync as an integrated feature, which others have pointed out, the other reason growth is limited is that filesystem storage and sync thereof has become less critical over time. Our apps increasingly do cloud-native walled-garden storage: docs in Google Docs / Notion / Confluence, mocks in Figma, etc. And code was cloud-native with version control much earlier.

I need sync for just photos on my phone (which Apple or Google are better for), and a small number of esigned PDFs and tax documents (for which any provider's free tier suffices).

Dropbox solved a problem of the 2010s.