Well, you add the google docs integration, or the dropbox integration, or the box integration, or use the Zapier integration to tie things together.

Slack isn't a file sharing platform, it's a communication platform. As such, they try to make communicating about, discovery and grouping of files in dedicated file sharing platforms easier.

That's a terrible solution for anyone handling data that has to remain on-prem, which is the case for a lot of large corporations that deal with financial data, personal id data, etc.

Of course it's a bad solution in that case. You've picked a case where a specific business requirement precludes using an external service. In which case, why would you use slack anyway, since it would then necessitate a chunk of your communication be external as well?