As code grows more complex over time (and into multiple modules), it is easy overwrite a global variable when you re-use the name by accident. And it can be hard to trace any problem, because you just can't put a debug print/stacktrace in the ONE function that is able to modify the variable.

Sometimes a couple of global variables might be the right choice, but more often than not they are a liability.

For me, the big exception to the rule is when i write code for microcontrollers. When you only have like 1000-4000 bytes of RAM (if you are lucky), ditching modern OO and basically laying out the memory map by hand is sometimes the best (and only) option. It's astonishing how much you can achieve with a kilobyte of memory when you spend a year shuffling bits and bytes around in your memory layout...