You’re referring to creating an intentional ground loop, I believe [https://en.m.wikipedia.org/wiki/Ground_loop_(electricity)].

The challenge as I understand it, is that yes you will get ‘free’ power (not actually free, as you had to create the low resistance electrical path for it to exist), but you have no control over the properties or values of what you get - and it will vary unpredictably.

It’s also unlikely you’ll consistently get much actual net power out of it, as you’re competing against an entire planets worth of reasonably conductive (in bulk) parallel paths.

It’s almost always a problem because of that.