Why is having many global variables a code smell?

A global is visible from anywhere in the translation unit, so any function can read or modify it. Bugs from globals are notoriously hard to track down because the "modifying" code can be physically far from the "reading" code, you can't tell where a value comes from without searching the whole file. Globals also make testing hard because functions become dependent on hidden state, so a test of f() might pass or fail based on what other functions ran before it. They make concurrency hard because two threads touching the same global require synchronization that's invisible at the call site. The cumulative effect is that a function's behavior stops being a function of its inputs alone, it depends on the entire history of the program. Local variables and explicit parameters force that dependency to show up in the code.