Open Source accounting software is one thing. Open Source that can automatically download and synchronize the transactions from all your bank accounts is very much another. In theory, there's a standardized file format and your bank(s) should expose an API to access it, but in practice it seems like banks really don't care about working with anybody smaller than Intuit and figuring out how to configure the connection for each bank is a software engineering task in and of itself.

Even Monarch Money (which seems to be the closest thing to a 1-to-1 replacement for Mint, since it's made by the original Mint developers) outsources the problem to several different "data providers" (e.g. Plaid, Finicity, MX) instead of trying to solve it in-house.

(All of the above applies to the US and maybe Canada. Europe is apparently entirely different (better?) in terms of standardization.)

By the way, GNUCash claims to be able to download using OFX, although I haven't had much success with it: https://wiki.gnucash.org/wiki/Setting_up_OFXDirectConnect