Replacing cartridge batteries is
a serious pain in the ass. You have to solder them in. And to make matters worse, most NES games use Gamebit screws, which require a Gamebit screwdriver, which you couldn't even buy until decades after the NES was obsolete, but they're cheap on Amazon now. And despite the batteries lasting a long time, they can get killed if you're not careful: NES was a janky system that always asked you to hold the Reset button while you pressed Power. This is because a small surge runs through the system and cartridge when you power it off. Not a problem with normal carts, but the NES wasn't designed from the start to deal with battery-backed saves. So, if you get unlucky and get a slightly more powerful surge than normal, that can toast your battery.