I think it makes sense that data would not be shared across accounts, especially by someone other than the account owner.
I also think the scenario of a musician who plays in multiple bands that play the same song in a similar enough way that it could use the same chart, but neither has added a chart, so that musician is in a position to want to add the same chart to multiple accounts, is relatively rare compared to the additional complexity that would be required to allow that.
I imagine this would come up when you're reviewing a band's set list before their gig. You see they have a song without a chart that you have a chart for, so you upload your own chart. Then some time later when you play another gig with another band and you see the same situation with the same song, you have to upload that same chart again. That doesn't seem harder than a new sharing capability, which would probably involve some type of navigating back to the original account, finding your previously uploaded charts, updating their sharing settings and then somehow linking them to the songs in the new account.
If you have a library of charts that you use in multiple bands, an alternative is just to maintain your own account, and as the band you're playing with at a given gig calls songs, pull up your own charts from your account. However, then you'd lose the ability to share their set lists or use live sharing if they're using that to choose songs on the fly, or view charts for songs they play that aren't in your library.