This comes from usability and consistency.

When we added the MSRV aware resolver, we decided to have it not affect `cargo install` because you could get an old version and not notice (especially with how noisy cargo is) when the right answer might be to upgrade.

Somewhat similar for min-publish-age but also we had defined the parent table for these config fields to not affect install.

The change to `--locked` made us feel a little better with this non-ideal circumstance.