You shouldn't just automatically trust it, but it allows you to examine what its doing and make your own informed decision about whether or not you can trust it. If you discover your data is being collected in an open source project you can, at the least, make an informed decision and give consent to it.

And while everyone won't be able to understand what they're looking at, the community as a whole would benefit from people looking at it an announcing/discussing problematic things they see in the code base.

If an open source project is syphoning data, someone's going to see it and talk about it.

If a closed project is doing it, it's harder / more complicated for that act to be discovered.

Because you can inspect the code and rebuild it to generate the same binaries.